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@ -27,6 +27,7 @@
11) What is configure all about? 11) What is configure all about?
12) How do I add a new port? 12) How do I add a new port?
13) What is CommandCounterIncrement()? 13) What is CommandCounterIncrement()?
13) Why don't we use threads in the backend?
_________________________________________________________________ _________________________________________________________________
1) What tools are available for developers? 1) What tools are available for developers?
@ -36,8 +37,7 @@
/tools directory are designed for developers. /tools directory are designed for developers.
RELEASE_CHANGES changes we have to make for each release RELEASE_CHANGES changes we have to make for each release
SQL_keywords standard SQL'92 keywords SQL_keywords standard SQL'92 keywords
backend description/flowchart of the backend directorie backend description/flowchart of the backend directories
s
ccsym find standard defines made by your compiler ccsym find standard defines made by your compiler
entab converts tabs to spaces, used by pgindent entab converts tabs to spaces, used by pgindent
find_static finds functions that could be made static find_static finds functions that could be made static
@ -95,7 +95,7 @@ s
emacs: emacs:
M-x set-variable tab-width M-x set-variable tab-width
or or
; Cmd to set tab stops &etc for working with PostgreSQL code ; Cmd to set tab stops & indenting for working with PostgreSQL code
(c-add-style "pgsql" (c-add-style "pgsql"
'("bsd" '("bsd"
(indent-tabs-mode . t) (indent-tabs-mode . t)
@ -108,8 +108,7 @@ s
and add this to your autoload list (modify file path in macro): and add this to your autoload list (modify file path in macro):
(setq auto-mode-alist (setq auto-mode-alist
(cons '("\\`/usr/local/src/pgsql/.*\\.[chyl]\\'" . pgsql- (cons '("\\`/usr/local/src/pgsql/.*\\.[chyl]\\'" . pgsql-c-mode)
c-mode)
auto-mode-alist)) auto-mode-alist))
or or
/* /*
@ -174,7 +173,6 @@ c-mode)
a typical code snipped that loops through a List containing Var a typical code snipped that loops through a List containing Var
*'s and processes each one: *'s and processes each one:
List *i, *list; List *i, *list;
foreach(i, list) foreach(i, list)
@ -207,7 +205,6 @@ c-mode)
You can print nodes easily inside gdb. First, to disable output You can print nodes easily inside gdb. First, to disable output
truncation when you use the gdb print command: truncation when you use the gdb print command:
(gdb) set print elements 0 (gdb) set print elements 0
Instead of printing values in gdb format, you can use the next two Instead of printing values in gdb format, you can use the next two
@ -215,7 +212,6 @@ c-mode)
format that is easier to understand. List's are unrolled into nodes, format that is easier to understand. List's are unrolled into nodes,
and nodes are printed in detail. The first prints in a short format, and nodes are printed in detail. The first prints in a short format,
and the second in a long format: and the second in a long format:
(gdb) call print(any_pointer) (gdb) call print(any_pointer)
(gdb) call pprint(any_pointer) (gdb) call pprint(any_pointer)
@ -311,8 +307,8 @@ c-mode)
9) How do I efficiently access information in tables from the backend code? 9) How do I efficiently access information in tables from the backend code?
You first need to find the tuples(rows) you are interested in. There You first need to find the tuples(rows) you are interested in. There
are two ways. First, SearchSysCache() and related functions allow are two ways. First, SearchSysCache() and related functions allow you
you to query the system catalogs. This is the preferred way to access to query the system catalogs. This is the preferred way to access
system tables, because the first call to the cache loads the needed system tables, because the first call to the cache loads the needed
rows, and future requests can return the results without accessing the rows, and future requests can return the results without accessing the
base table. The caches use system table indexes to look up tuples. A base table. The caches use system table indexes to look up tuples. A
@ -321,13 +317,14 @@ c-mode)
src/backend/utils/cache/lsyscache.c contains many column-specific src/backend/utils/cache/lsyscache.c contains many column-specific
cache lookup functions. cache lookup functions.
The rows returned are cache-owned versions of the heap rows. Therefore, The rows returned are cache-owned versions of the heap rows.
you must not modify or delete the tuple returned by SearchSysCache(). Therefore, you must not modify or delete the tuple returned by
What you *should* do is release it with ReleaseSysCache() when you are SearchSysCache(). What you should do is release it with
done using it; this informs the cache that it can discard that tuple ReleaseSysCache() when you are done using it; this informs the cache
if necessary. If you neglect to call ReleaseSysCache(), then the cache that it can discard that tuple if necessary. If you neglect to call
entry will remain locked in the cache until end of transaction, which is ReleaseSysCache(), then the cache entry will remain locked in the
tolerable but not very desirable. cache until end of transaction, which is tolerable but not very
desirable.
If you can't use the system cache, you will need to retrieve the data If you can't use the system cache, you will need to retrieve the data
directly from the heap table, using the buffer cache that is shared by directly from the heap table, using the buffer cache that is shared by
@ -345,27 +342,25 @@ c-mode)
heap_fetch(), you must pass a Buffer pointer, and ReleaseBuffer() it heap_fetch(), you must pass a Buffer pointer, and ReleaseBuffer() it
when completed. when completed.
Once you have the row, you can get data that is common Once you have the row, you can get data that is common to all tuples,
to all tuples, like t_self and t_oid, by merely accessing the like t_self and t_oid, by merely accessing the HeapTuple structure
HeapTuple structure entries. If you need a table-specific column, you entries. If you need a table-specific column, you should take the
should take the HeapTuple pointer, and use the GETSTRUCT() macro to HeapTuple pointer, and use the GETSTRUCT() macro to access the
access the table-specific start of the tuple. You then cast the table-specific start of the tuple. You then cast the pointer as a
pointer as a Form_pg_proc pointer if you are accessing the pg_proc Form_pg_proc pointer if you are accessing the pg_proc table, or
table, or Form_pg_type if you are accessing pg_type. You can then Form_pg_type if you are accessing pg_type. You can then access the
access the columns by using a structure pointer: columns by using a structure pointer:
((Form_pg_class) GETSTRUCT(tuple))->relnatts ((Form_pg_class) GETSTRUCT(tuple))->relnatts
You must not directly change live tuples in this way. The best way You must not directly change live tuples in this way. The best way is
is to use heap_modifytuple() and pass it your original tuple, and the to use heap_modifytuple() and pass it your original tuple, and the
values you want changed. It returns a palloc'ed tuple, which you values you want changed. It returns a palloc'ed tuple, which you pass
pass to heap_replace(). You can delete tuples by passing the tuple's to heap_replace(). You can delete tuples by passing the tuple's t_self
t_self to heap_destroy(). You use t_self for heap_update() too. to heap_destroy(). You use t_self for heap_update() too. Remember,
tuples can be either system cache copies, which may go away after you
Remember, tuples can be either system cache copies, which may go away call ReleaseSysCache(), or read directly from disk buffers, which go
after you call ReleaseSysCache(), or read directly from disk buffers, away when you heap_getnext(), heap_endscan, or ReleaseBuffer(), in the
which go away when you heap_getnext(), heap_endscan, or ReleaseBuffer(), heap_fetch() case. Or it may be a palloc'ed tuple, that you must
in the heap_fetch() case. Or it may be a palloc'ed tuple, that you must
pfree() when finished. pfree() when finished.
10) What is elog()? 10) What is elog()?
@ -429,3 +424,12 @@ c-mode)
to be broken into pieces so each piece can see rows modified by to be broken into pieces so each piece can see rows modified by
previous pieces. CommandCounterIncrement() increments the Command previous pieces. CommandCounterIncrement() increments the Command
Counter, creating a new part of the transaction. Counter, creating a new part of the transaction.
14) Why don't we use threads in the backend?
There are several reasons threads are not used:
* Historically, threads were unsupported and buggy.
* An error in one backend can corrupt other backends.
* Speed improvements using threads are small compared to the
remaining backend startup time.
* The backend code would be more complex.

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@ -1,54 +1,66 @@
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN">
<HTML> <HTML>
<HEAD> <HEAD>
<TITLE>PostgreSQL Developers FAQ</title> <META name="generator" content="HTML Tidy, see www.w3.org">
<TITLE>PostgreSQL Developers FAQ</TITLE>
</HEAD> </HEAD>
<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#FF0000" VLINK="#A00000" ALINK="#0000FF">
<H1> <BODY bgcolor="#FFFFFF" text="#000000" link="#FF0000" vlink="#A00000"
Developer's Frequently Asked Questions (FAQ) for PostgreSQL alink="#0000FF">
</H1> <H1>Developer's Frequently Asked Questions (FAQ) for
<P> PostgreSQL</H1>
Last updated: Fri Jun 9 21:54:54 EDT 2000
<P> <P>Last updated: Fri Jun 9 21:54:54 EDT 2000</P>
Current maintainer: Bruce Momjian (<a
href="mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</a>)<BR> <P>Current maintainer: Bruce Momjian (<A href=
<P> "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
The most recent version of this document can be viewed at </P>
the postgreSQL Web site, <a
href="http://PostgreSQL.org">http://PostgreSQL.org</a>. <P>The most recent version of this document can be viewed at the
<P> postgreSQL Web site, <A href=
"http://PostgreSQL.org">http://PostgreSQL.org</A>.<BR>
</P>
<HR> <HR>
<P> <BR>
<CENTER><H2>Questions</H2></CENTER> <CENTER>
<a href="#1">1</a>) What tools are available for developers?<BR> <H2>Questions</H2>
<a href="#2">2</a>) What books are good for developers?<BR> </CENTER>
<a href="#3">3</a>) Why do we use <I>palloc</I>() and <I>pfree</I>() to allocate memory?<BR> <A href="#1">1</A>) What tools are available for developers?<BR>
<a href="#4">4</a>) Why do we use <I>Node</I> and <I>List</I> to <A href="#2">2</A>) What books are good for developers?<BR>
<A href="#3">3</A>) Why do we use <I>palloc</I>() and
<I>pfree</I>() to allocate memory?<BR>
<A href="#4">4</A>) Why do we use <I>Node</I> and <I>List</I> to
make data structures?<BR> make data structures?<BR>
<a href="#5">5</a>) How do I add a feature or fix a bug?<BR> <A href="#5">5</A>) How do I add a feature or fix a bug?<BR>
<a href="#6">6</a>) How do I download/update the current source tree?<BR> <A href="#6">6</A>) How do I download/update the current source
<a href="#7">7</a>) How do I test my changes?<BR> tree?<BR>
<a href="#7">7</a>) I just added a field to a structure. What else <A href="#7">7</A>) How do I test my changes?<BR>
<A href="#7">7</A>) I just added a field to a structure. What else
should I do?<BR> should I do?<BR>
<a href="#8">8</a>) Why are table, column, type, function, view <A href="#8">8</A>) Why are table, column, type, function, view
names sometimes referenced as <I>Name</I> or <I>NameData,</I> and names sometimes referenced as <I>Name</I> or <I>NameData,</I> and
sometimes as <I>char *?</I><BR> sometimes as <I>char *?</I><BR>
<a href="#9">9</a>) How do I efficiently access information in <A href="#9">9</A>) How do I efficiently access information in
tables from the backend code?<BR> tables from the backend code?<BR>
<a href="#10">10</a>) What is elog()?<BR> <A href="#10">10</A>) What is elog()?<BR>
<a href="#11">11</a>) What is configure all about?<BR> <A href="#11">11</A>) What is configure all about?<BR>
<a href="#12">12</a>) How do I add a new port?<BR> <A href="#12">12</A>) How do I add a new port?<BR>
<a href="#13">13</a>) What is CommandCounterIncrement()?<BR> <A href="#13">13</A>) What is CommandCounterIncrement()?<BR>
<A href="#14">13</A>) Why don't we use threads in the backend?<BR>
<BR> <BR>
<HR>
<H3><a <HR>
name="1">1</a>) What tools are available for developers?</H3><P>
Aside from the User documentation mentioned in the regular FAQ, there <H3><A name="1">1</A>) What tools are available for
are several development tools available. First, all the files in the developers?</H3>
<I>/tools</I> directory are designed for developers.
<P>Aside from the User documentation mentioned in the regular FAQ,
there are several development tools available. First, all the files
in the <I>/tools</I> directory are designed for developers.</P>
<PRE> <PRE>
RELEASE_CHANGES changes we have to make for each release RELEASE_CHANGES changes we have to make for each release
SQL_keywords standard SQL'92 keywords SQL_keywords standard SQL'92 keywords
@ -67,47 +79,45 @@ are several development tools available. First, all the files in the
pginclude scripts for adding/removing include files pginclude scripts for adding/removing include files
unused_oids in pgsql/src/include/catalog unused_oids in pgsql/src/include/catalog
</PRE> </PRE>
Let me note some of these. If you point your browser at the Let me note some of these. If you point your browser at the
<I>file:/usr/local/src/pgsql/src/tools/backend/index.html</I> directory, <I>file:/usr/local/src/pgsql/src/tools/backend/index.html</I>
you will see few paragraphs describing the data flow, the backend directory, you will see few paragraphs describing the data flow,
components in a flow chart, and a description of the shared memory area. the backend components in a flow chart, and a description of the
You can click on any flowchart box to see a description. If you then shared memory area. You can click on any flowchart box to see a
click on the directory name, you will be taken to the source directory, description. If you then click on the directory name, you will be
to browse the actual source code behind it. We also have several README taken to the source directory, to browse the actual source code
files in some source directories to describe the function of the module. behind it. We also have several README files in some source
The browser will display these when you enter the directory also. The directories to describe the function of the module. The browser
<I>tools/backend</I> directory is also contained on our web page under will display these when you enter the directory also. The
the title <I>How PostgreSQL Processes a Query.</I><P> <I>tools/backend</I> directory is also contained on our web page
under the title <I>How PostgreSQL Processes a Query.</I>
Second, you really should have an editor that can handle tags, so you <P>Second, you really should have an editor that can handle tags,
can tag a function call to see the function definition, and then tag so you can tag a function call to see the function definition, and
inside that function to see an even lower-level function, and then back then tag inside that function to see an even lower-level function,
out twice to return to the original function. Most editors support this and then back out twice to return to the original function. Most
via <I>tags</I> or <I>etags</I> files.<P> editors support this via <I>tags</I> or <I>etags</I> files.</P>
<P>Third, you need to get <I>id-utils</I> from:</P>
Third, you need to get <I>id-utils</I> from: <PRE>
<pre> <A href=
<a href="ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz">ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz</a> "ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz">ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz</A>
<a href="ftp://tug.org/gnu/id-utils-3.2d.tar.gz">ftp://tug.org/gnu/id-utils-3.2d.tar.gz</a> <A href=
<a href="ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz">ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz</a> "ftp://tug.org/gnu/id-utils-3.2d.tar.gz">ftp://tug.org/gnu/id-utils-3.2d.tar.gz</A>
</pre> <A href=
"ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz">ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz</A>
By running <I>tools/make_mkid</I>, an archive of source symbols can be </PRE>
created that can be rapidly queried like <I>grep</I> or edited. Others By running <I>tools/make_mkid</I>, an archive of source symbols can
prefer <I>glimpse.</I><P> be created that can be rapidly queried like <I>grep</I> or edited.
Others prefer <I>glimpse.</I>
<I>make_diff</I> has tools to create patch diff files that can be
applied to the distribution.<P>
<P><I>make_diff</I> has tools to create patch diff files that can
be applied to the distribution.</P>
Our standard format is to indent each code level with one tab, where <P>Our standard format is to indent each code level with one tab,
each tab is four spaces. You will need to set your editor to display where each tab is four spaces. You will need to set your editor to
tabs as four spaces: display tabs as four spaces:<BR>
<BR> </P>
<PRE> <PRE>
vi in ~/.exrc: vi in ~/.exrc:
set tabstop=4 set tabstop=4
@ -119,7 +129,7 @@ tabs as four spaces:
emacs: emacs:
M-x set-variable tab-width M-x set-variable tab-width
or or
; Cmd to set tab stops &etc for working with PostgreSQL code ; Cmd to set tab stops &amp; indenting for working with PostgreSQL code
(c-add-style "pgsql" (c-add-style "pgsql"
'("bsd" '("bsd"
(indent-tabs-mode . t) (indent-tabs-mode . t)
@ -144,70 +154,76 @@ tabs as four spaces:
*/ */
</PRE> </PRE>
<BR> <BR>
<I>pgindent</I> will the format code by specifying <I>pgindent</I> will the format code by specifying flags to your
flags to your operating system's utility <I>indent.</I><P> operating system's utility <I>indent.</I>
<I>pgindent</I> is run on all source files just before each beta test
period. It auto-formats all source files to make them consistent. <P><I>pgindent</I> is run on all source files just before each beta
Comment blocks that need specific line breaks should be formatted as test period. It auto-formats all source files to make them
<I>block comments,</I> where the comment starts as consistent. Comment blocks that need specific line breaks should be
<CODE>/*------</CODE>. These comments will not be reformatted in any formatted as <I>block comments,</I> where the comment starts as
way. <CODE>/*------</CODE>. These comments will not be reformatted in
any way. <I>pginclude</I> contains scripts used to add needed
<I>pginclude</I> contains scripts used to add needed #include's to #include's to include files, and removed unneeded #include's. When
include files, and removed unneeded #include's. adding system types, you will need to assign oids to them. There is
also a script called <I>unused_oids</I> in
When adding system types, you will need to assign oids to them. <I>pgsql/src/include/catalog</I> that shows the unused oids.</P>
There is also a script called <I>unused_oids</I> in
<I>pgsql/src/include/catalog</I> that shows the unused oids. <H3><A name="2">2</A>) What books are good for developers?</H3>
<H3><a name="2">2</a>) What books are good for developers?</H3><P> <P>I have four good books, <I>An Introduction to Database
Systems,</I> by C.J. Date, Addison, Wesley, <I>A Guide to the SQL
I have four good books, <I>An Introduction to Database Systems,</I> by Standard,</I> by C.J. Date, et. al, Addison, Wesley,
C.J. Date, Addison, Wesley, <I>A Guide to the SQL Standard,</I> by C.J. <I>Fundamentals of Database Systems,</I> by Elmasri and Navathe,
Date, et. al, Addison, Wesley, <I>Fundamentals of Database Systems,</I> and <I>Transaction Processing,</I> by Jim Gray, Morgan,
by Elmasri and Navathe, and <I>Transaction Processing,</I> by Jim Gray, Kaufmann</P>
Morgan, Kaufmann<P>
<P>There is also a database performance site, with a handbook
There is also a database performance site, with a handbook on-line on-line written by Jim Gray at <A href=
written by Jim Gray at <A "http://www.benchmarkresources.com">http://www.benchmarkresources.com.</A></P>
HREF="http://www.benchmarkresources.com">http://www.benchmarkresources.com.</A>
<H3><A name="3">3</A>) Why do we use <I>palloc</I>() and
<I>pfree</I>() to allocate memory?</H3>
<P><I>palloc()</I> and <I>pfree()</I> are used in place of malloc()
and free() because we automatically free all memory allocated when
a transaction completes. This makes it easier to make sure we free
memory that gets allocated in one place, but only freed much later.
There are several contexts that memory can be allocated in, and
this controls when the allocated memory is automatically freed by
the backend.</P>
<H3><A name="4">4</A>) Why do we use <I>Node</I> and <I>List</I> to
make data structures?</H3>
<P>We do this because this allows a consistent way to pass data
inside the backend in a flexible way. Every node has a
<I>NodeTag</I> which specifies what type of data is inside the
Node. <I>Lists</I> are groups of <I>Nodes chained together as a
forward-linked list.</I></P>
<P>Here are some of the <I>List</I> manipulation commands:</P>
<BLOCKQUOTE>
<DL>
<DT>lfirst(i)</DT>
<H3><a name="3">3</a>) Why do we use <I>palloc</I>() and <I>pfree</I>() <DD>return the data at list element <I>i.</I></DD>
to allocate memory?</H3><P>
<I>palloc()</I> and <I>pfree()</I> are used in place of malloc() and <DT>lnext(i)</DT>
free() because we automatically free all memory allocated when a
transaction completes. This makes it easier to make sure we free memory
that gets allocated in one place, but only freed much later. There are
several contexts that memory can be allocated in, and this controls when
the allocated memory is automatically freed by the backend.<P>
<DD>return the next list element after <I>i.</I></DD>
<H3><a name="4">4</a>) Why do we use <I>Node</I> and <I>List</I> to <DT>foreach(i, list)</DT>
make data structures?</H3><P>
We do this because this allows a consistent way to pass data inside the <DD>
backend in a flexible way. Every node has a <I>NodeTag</I> which loop through <I>list,</I> assigning each list element to
specifies what type of data is inside the Node. <I>Lists</I> are groups <I>i.</I> It is important to note that <I>i</I> is a List *,
of <I>Nodes chained together as a forward-linked list.</I><P> not the data in the <I>List</I> element. You need to use
Here are some of the <I>List</I> manipulation commands: <I>lfirst(i)</I> to get at the data. Here is a typical code
<BLOCKQUOTE> snipped that loops through a List containing <I>Var *'s</I>
<DL> and processes each one:
<DT>lfirst(i)
<DD>return the data at list element <I>i.</I>
<DT>lnext(i)
<DD>return the next list element after <I>i.</I>
<DT>foreach(i, list)
<DD>loop through <I>list,</I> assigning each list element to <I>i.</I>
It is important to note that <I>i</I> is a List *, not the data in the
<I>List</I> element. You need to use <I>lfirst(i)</I> to get at the data.
Here is a typical code snipped that loops through a List containing
<I>Var *'s</I> and processes each one:
<PRE> <PRE>
<CODE> <CODE>List *i, *list;
List *i, *list;
foreach(i, list) foreach(i, list)
{ {
@ -217,281 +233,306 @@ Here is a typical code snipped that loops through a List containing
} }
</CODE> </CODE>
</PRE> </PRE>
<DT>lcons(node, list) </DD>
<DD>add <I>node</I> to the front of <I>list,</I> or create a new list with
<I>node</I> if <I>list</I> is <I>NIL.</I> <DT>lcons(node, list)</DT>
<DT>lappend(list, node)
<DD>add <I>node</I> to the end of <I>list.</I> This is more expensive <DD>add <I>node</I> to the front of <I>list,</I> or create a
that lcons. new list with <I>node</I> if <I>list</I> is <I>NIL.</I></DD>
<DT>nconc(list1, list2)
<DD>Concat <I>list2</I> on to the end of <I>list1.</I> <DT>lappend(list, node)</DT>
<DT>length(list)
<DD>return the length of the <I>list.</I> <DD>add <I>node</I> to the end of <I>list.</I> This is more
<DT>nth(i, list) expensive that lcons.</DD>
<DD>return the <I>i</I>'th element in <I>list.</I>
<DT>lconsi, ... <DT>nconc(list1, list2)</DT>
<DD>There are integer versions of these: <I>lconsi, lappendi, nthi.</I>
<I>List's</I> containing integers instead of Node pointers are used to <DD>Concat <I>list2</I> on to the end of <I>list1.</I></DD>
hold list of relation object id's and other integer quantities.
<DT>length(list)</DT>
<DD>return the length of the <I>list.</I></DD>
<DT>nth(i, list)</DT>
<DD>return the <I>i</I>'th element in <I>list.</I></DD>
<DT>lconsi, ...</DT>
<DD>There are integer versions of these: <I>lconsi, lappendi,
nthi.</I> <I>List's</I> containing integers instead of Node
pointers are used to hold list of relation object id's and
other integer quantities.</DD>
</DL> </DL>
</BLOCKQUOTE> </BLOCKQUOTE>
You can print nodes easily inside <I>gdb.</I> First, to disable You can print nodes easily inside <I>gdb.</I> First, to disable
output truncation when you use the gdb <I>print</I> command: output truncation when you use the gdb <I>print</I> command:
<PRE> <PRE>
<CODE> <CODE>(gdb) set print elements 0
(gdb) set print elements 0
</CODE> </CODE>
</PRE> </PRE>
Instead of printing values in gdb format, you can use the next two Instead of printing values in gdb format, you can use the next two
commands to print out List, Node, and structure contents in a verbose commands to print out List, Node, and structure contents in a
format that is easier to understand. List's are unrolled into nodes, verbose format that is easier to understand. List's are unrolled
and nodes are printed in detail. The first prints in a short format, into nodes, and nodes are printed in detail. The first prints in a
and the second in a long format: short format, and the second in a long format:
<PRE> <PRE>
<CODE> <CODE>(gdb) call print(any_pointer)
(gdb) call print(any_pointer)
(gdb) call pprint(any_pointer) (gdb) call pprint(any_pointer)
</CODE> </CODE>
</PRE> </PRE>
The output appears in the postmaster log file, or on your screen if you The output appears in the postmaster log file, or on your screen if
are running a backend directly without a postmaster. you are running a backend directly without a postmaster.
<P>
<H3><a name="5">5</a>) How do I add a feature or fix a bug?</H3><P>
The source code is over 250,000 lines. Many problems/features are <H3><A name="5">5</A>) How do I add a feature or fix a bug?</H3>
isolated to one specific area of the code. Others require knowledge of
much of the source. If you are confused about where to start, ask the
hackers list, and they will be glad to assess the complexity and give
pointers on where to start.<P>
Another thing to keep in mind is that many fixes and features can be <P>The source code is over 250,000 lines. Many problems/features
added with surprisingly little code. I often start by adding code, then are isolated to one specific area of the code. Others require
looking at other areas in the code where similar things are done, and by knowledge of much of the source. If you are confused about where to
the time I am finished, the patch is quite small and compact.<P> start, ask the hackers list, and they will be glad to assess the
complexity and give pointers on where to start.</P>
When adding code, keep in mind that it should use the existing <P>Another thing to keep in mind is that many fixes and features
facilities in the source, for performance reasons and for simplicity. can be added with surprisingly little code. I often start by adding
Often a review of existing code doing similar things is helpful.<P> code, then looking at other areas in the code where similar things
are done, and by the time I am finished, the patch is quite small
and compact.</P>
<P>When adding code, keep in mind that it should use the existing
facilities in the source, for performance reasons and for
simplicity. Often a review of existing code doing similar things is
helpful.</P>
<H3><a name="6">6</a>) How do I download/update the current source <H3><A name="6">6</A>) How do I download/update the current source
tree?</H3><P> tree?</H3>
<P>There are several ways to obtain the source tree. Occasional
There are several ways to obtain the source tree. Occasional developers developers can just get the most recent source tree snapshot from
can just get the most recent source tree snapshot from
ftp.postgresql.org. For regular developers, you can use CVS. CVS ftp.postgresql.org. For regular developers, you can use CVS. CVS
allows you to download the source tree, then occasionally update your allows you to download the source tree, then occasionally update
copy of the source tree with any new changes. Using CVS, you don't have your copy of the source tree with any new changes. Using CVS, you
to download the entire source each time, only the changed files. don't have to download the entire source each time, only the
Anonymous CVS does not allows developers to update the remote source changed files. Anonymous CVS does not allows developers to update
tree, though privileged developers can do this. There is a CVS FAQ on the remote source tree, though privileged developers can do this.
our web site that describes how to use remote CVS. You can also use There is a CVS FAQ on our web site that describes how to use remote
CVSup, which has similarly functionality, and is available from CVS. You can also use CVSup, which has similarly functionality, and
ftp.postgresql.org.<P> is available from ftp.postgresql.org.</P>
To update the source tree, there are two ways. You can generate a patch <P>To update the source tree, there are two ways. You can generate
against your current source tree, perhaps using the make_diff tools a patch against your current source tree, perhaps using the
mentioned above, and send them to the patches list. They will be make_diff tools mentioned above, and send them to the patches list.
reviewed, and applied in a timely manner. If the patch is major, and we They will be reviewed, and applied in a timely manner. If the patch
are in beta testing, the developers may wait for the final release is major, and we are in beta testing, the developers may wait for
before applying your patches.<P> the final release before applying your patches.</P>
For hard-core developers, Marc(scrappy@postgresql.org) will give you a <P>For hard-core developers, Marc(scrappy@postgresql.org) will give
Unix shell account on postgresql.org, so you can use CVS to update the you a Unix shell account on postgresql.org, so you can use CVS to
main source tree, or you can ftp your files into your account, patch, update the main source tree, or you can ftp your files into your
and cvs install the changes directly into the source tree. <P> account, patch, and cvs install the changes directly into the
source tree.</P>
<H3><a name="6">6</a>) How do I test my changes?</H3><P>
<H3><A name="6">6</A>) How do I test my changes?</H3>
First, use <I>psql</I> to make sure it is working as you expect. Then
run <I>src/test/regress</I> and get the output of <P>First, use <I>psql</I> to make sure it is working as you expect.
<I>src/test/regress/checkresults</I> with and without your changes, to Then run <I>src/test/regress</I> and get the output of
see that your patch does not change the regression test in unexpected <I>src/test/regress/checkresults</I> with and without your changes,
ways. This practice has saved me many times. The regression tests test to see that your patch does not change the regression test in
the code in ways I would never do, and has caught many bugs in my unexpected ways. This practice has saved me many times. The
patches. By finding the problems now, you save yourself a lot of regression tests test the code in ways I would never do, and has
debugging later when things are broken, and you can't figure out when it caught many bugs in my patches. By finding the problems now, you
happened.<P> save yourself a lot of debugging later when things are broken, and
you can't figure out when it happened.</P>
<H3><a name="7">7</a>) I just added a field to a structure. What else <H3><A name="7">7</A>) I just added a field to a structure. What
should I do?</H3><P> else should I do?</H3>
The structures passing around from the parser, rewrite, optimizer, and <P>The structures passing around from the parser, rewrite,
executor require quite a bit of support. Most structures have support optimizer, and executor require quite a bit of support. Most
routines in <I>src/backend/nodes</I> used to create, copy, read, and output structures have support routines in <I>src/backend/nodes</I> used
those structures. Make sure you add support for your new field to these to create, copy, read, and output those structures. Make sure you
files. Find any other places the structure may need code for your new add support for your new field to these files. Find any other
field. <I>mkid</I> is helpful with this (see above).<P> places the structure may need code for your new field. <I>mkid</I>
is helpful with this (see above).</P>
<H3><a name="8">8</a>) Why are table, column, type, function, view <H3><A name="8">8</A>) Why are table, column, type, function, view
names sometimes referenced as <I>Name</I> or <I>NameData,</I> and names sometimes referenced as <I>Name</I> or <I>NameData,</I> and
sometimes as <I>char *?</I></H3><P> sometimes as <I>char *?</I></H3>
Table, column, type, function, and view names are stored in system
tables in columns of type <I>Name.</I> Name is a fixed-length,
null-terminated type of <I>NAMEDATALEN</I> bytes. (The default value
for NAMEDATALEN is 32 bytes.)
<PRE><CODE> <P>Table, column, type, function, and view names are stored in
typedef struct nameData system tables in columns of type <I>Name.</I> Name is a
fixed-length, null-terminated type of <I>NAMEDATALEN</I> bytes.
(The default value for NAMEDATALEN is 32 bytes.)</P>
<PRE>
<CODE>typedef struct nameData
{ {
char data[NAMEDATALEN]; char data[NAMEDATALEN];
} NameData; } NameData;
typedef NameData *Name; typedef NameData *Name;
</CODE></PRE> </CODE>
</PRE>
Table, column, type, function, and view names that come into the Table, column, type, function, and view names that come into the
backend via user queries are stored as variable-length, null-terminated backend via user queries are stored as variable-length,
character strings.<P> null-terminated character strings.
Many functions are called with both types of names, ie. <I>heap_open().</I> <P>Many functions are called with both types of names, ie.
Because the Name type is null-terminated, it is safe to pass it to a <I>heap_open().</I> Because the Name type is null-terminated, it is
function expecting a char *. Because there are many cases where on-disk safe to pass it to a function expecting a char *. Because there are
names(Name) are compared to user-supplied names(char *), there are many many cases where on-disk names(Name) are compared to user-supplied
cases where Name and char * are used interchangeably.<P> names(char *), there are many cases where Name and char * are used
interchangeably.</P>
<H3><a name="9">9</a>) How do I efficiently access information in
tables from the backend code?</H3><P> <H3><A name="9">9</A>) How do I efficiently access information in
tables from the backend code?</H3>
You first need to find the tuples(rows) you are interested in. There
are two ways. First, <I>SearchSysCache()</I> and related functions <P>You first need to find the tuples(rows) you are interested in.
allow you to query the system catalogs. This is the preferred way to There are two ways. First, <I>SearchSysCache()</I> and related
access system tables, because the first call to the cache loads the functions allow you to query the system catalogs. This is the
needed rows, and future requests can return the results without preferred way to access system tables, because the first call to
accessing the base table. The caches use system table indexes the cache loads the needed rows, and future requests can return the
to look up tuples. A list of available caches is located in results without accessing the base table. The caches use system
<I>src/backend/utils/cache/syscache.c.</I> table indexes to look up tuples. A list of available caches is
<I>src/backend/utils/cache/lsyscache.c</I> contains many column-specific located in <I>src/backend/utils/cache/syscache.c.</I>
cache lookup functions.<P> <I>src/backend/utils/cache/lsyscache.c</I> contains many
column-specific cache lookup functions.</P>
The rows returned are cache-owned versions of the heap rows. Therefore, you
must not modify or delete the tuple returned by <I>SearchSysCache()</I>. What <P>The rows returned are cache-owned versions of the heap rows.
you <I>should</I> do is release it with <I>ReleaseSysCache()</I> when you are Therefore, you must not modify or delete the tuple returned by
done using it; this informs the cache that it can discard that tuple if <I>SearchSysCache()</I>. What you <I>should</I> do is release it
necessary. If you neglect to call <I>ReleaseSysCache()</I>, then the cache with <I>ReleaseSysCache()</I> when you are done using it; this
entry will remain locked in the cache until end of transaction, which is informs the cache that it can discard that tuple if necessary. If
tolerable but not very desirable.<P> you neglect to call <I>ReleaseSysCache()</I>, then the cache entry
will remain locked in the cache until end of transaction, which is
If you can't use the system cache, you will need to retrieve the data tolerable but not very desirable.</P>
directly from the heap table, using the buffer cache that is shared by
all backends. The backend automatically takes care of loading the rows <P>If you can't use the system cache, you will need to retrieve the
into the buffer cache.<P> data directly from the heap table, using the buffer cache that is
shared by all backends. The backend automatically takes care of
Open the table with <I>heap_open().</I> You can then start a table scan loading the rows into the buffer cache.</P>
with <I>heap_beginscan(),</I> then use <I>heap_getnext()</I> and
continue as long as <I>HeapTupleIsValid()</I> returns true. Then do a <P>Open the table with <I>heap_open().</I> You can then start a
<I>heap_endscan().</I> <I>Keys</I> can be assigned to the <I>scan.</I> table scan with <I>heap_beginscan(),</I> then use
No indexes are used, so all rows are going to be compared to the keys, <I>heap_getnext()</I> and continue as long as
and only the valid rows returned.<P> <I>HeapTupleIsValid()</I> returns true. Then do a
<I>heap_endscan().</I> <I>Keys</I> can be assigned to the
You can also use <I>heap_fetch()</I> to fetch rows by block <I>scan.</I> No indexes are used, so all rows are going to be
compared to the keys, and only the valid rows returned.</P>
<P>You can also use <I>heap_fetch()</I> to fetch rows by block
number/offset. While scans automatically lock/unlock rows from the number/offset. While scans automatically lock/unlock rows from the
buffer cache, with <I>heap_fetch(),</I> you must pass a <I>Buffer</I> buffer cache, with <I>heap_fetch(),</I> you must pass a
pointer, and <I>ReleaseBuffer()</I> it when completed.<P> <I>Buffer</I> pointer, and <I>ReleaseBuffer()</I> it when
completed.</P>
Once you have the row, you can get data that is common to all tuples,
like <I>t_self</I> and <I>t_oid,</I> by merely accessing the <P>Once you have the row, you can get data that is common to all
<I>HeapTuple</I> structure entries. tuples, like <I>t_self</I> and <I>t_oid,</I> by merely accessing
If you need a table-specific column, you should take the HeapTuple the <I>HeapTuple</I> structure entries. If you need a
pointer, and use the <I>GETSTRUCT()</I> macro to access the table-specific column, you should take the HeapTuple pointer, and
table-specific start of the tuple. You then cast the pointer as a use the <I>GETSTRUCT()</I> macro to access the table-specific start
<I>Form_pg_proc</I> pointer if you are accessing the pg_proc table, or of the tuple. You then cast the pointer as a <I>Form_pg_proc</I>
<I>Form_pg_type</I> if you are accessing pg_type. You can then access pointer if you are accessing the pg_proc table, or
the columns by using a structure pointer: <I>Form_pg_type</I> if you are accessing pg_type. You can then
access the columns by using a structure pointer:</P>
<PRE> <PRE>
<CODE> <CODE>((Form_pg_class) GETSTRUCT(tuple))-&gt;relnatts
((Form_pg_class) GETSTRUCT(tuple))-&gt;relnatts
</CODE> </CODE>
</PRE> </PRE>
You must not directly change <I>live</I> tuples in this way. The
You must not directly change <I>live</I> tuples in this way. The best best way is to use <I>heap_modifytuple()</I> and pass it your
way is to use <I>heap_modifytuple()</I> and pass it your original original tuple, and the values you want changed. It returns a
tuple, and the values you want changed. It returns a palloc'ed palloc'ed tuple, which you pass to <I>heap_replace().</I> You can
tuple, which you pass to <I>heap_replace().</I> delete tuples by passing the tuple's <I>t_self</I> to
<I>heap_destroy().</I> You use <I>t_self</I> for
You can delete tuples by passing the tuple's <I>t_self</I> to <I>heap_update()</I> too. Remember, tuples can be either system
<I>heap_destroy().</I> You use <I>t_self</I> for <I>heap_update()</I> too. cache copies, which may go away after you call
<I>ReleaseSysCache()</I>, or read directly from disk buffers, which
Remember, tuples can be either system cache copies, which may go away after
you call <I>ReleaseSysCache()</I>, or read directly from disk buffers, which
go away when you <I>heap_getnext()</I>, <I>heap_endscan</I>, or go away when you <I>heap_getnext()</I>, <I>heap_endscan</I>, or
<I>ReleaseBuffer()</I>, in the <I>heap_fetch()</I> case. Or it may be a <I>ReleaseBuffer()</I>, in the <I>heap_fetch()</I> case. Or it may
palloc'ed tuple, that you must <I>pfree()</I> when finished. be a palloc'ed tuple, that you must <I>pfree()</I> when finished.
<H3><a name="10">10</a>) What is elog()?</H3><P> <H3><A name="10">10</A>) What is elog()?</H3>
<I>elog()</I> is used to send messages to the front-end, and optionally <P><I>elog()</I> is used to send messages to the front-end, and
terminate the current query being processed. The first parameter is an optionally terminate the current query being processed. The first
elog level of <I>NOTICE,</I> <I>DEBUG,</I> <I>ERROR,</I> or parameter is an elog level of <I>NOTICE,</I> <I>DEBUG,</I>
<I>FATAL.</I> <I>ERROR,</I> or <I>FATAL.</I> <I>NOTICE</I> prints on the user's
terminal and the postmaster logs. <I>DEBUG</I> prints only in the
<I>NOTICE</I> prints on the user's terminal and the postmaster logs. postmaster logs. <I>ERROR</I> prints in both places, and terminates
<I>DEBUG</I> prints only in the postmaster logs. <I>ERROR</I> prints in the current query, never returning from the call. <I>FATAL</I>
both places, and terminates the current query, never returning from the call. terminates the backend process. The remaining parameters of
<I>FATAL</I> terminates the backend process. <I>elog</I> are a <I>printf</I>-style set of parameters to
print.</P>
The remaining parameters of <I>elog</I> are a <I>printf</I>-style set of
parameters to print. <H3><A name="11">11</A>) What is configure all about?</H3>
<H3><a name="11">11</a>) What is configure all about?</H3><P> <P>The files <I>configure</I> and <I>configure.in</I> are part of
the GNU <I>autoconf</I> package. Configure allows us to test for
The files <I>configure</I> and <I>configure.in</I> are part of the various capabilities of the OS, and to set variables that can then
GNU <I>autoconf</I> package. Configure allows us to test for various be tested in C programs and Makefiles. Autoconf is installed on the
capabilities of the OS, and to set variables that can then be tested in PostgreSQL main server. To add options to configure, edit
C programs and Makefiles. Autoconf is installed on the PostgreSQL main <I>configure.in,</I> and then run <I>autoconf</I> to generate
server. To add options to configure, edit <I>configure.in,</I> and then <I>configure.</I></P>
run <I>autoconf</I> to generate <I>configure.</I><P>
<P>When <I>configure</I> is run by the user, it tests various OS
When <I>configure</I> is run by the user, it tests various OS
capabilities, stores those in <I>config.status</I> and capabilities, stores those in <I>config.status</I> and
<I>config.cache,</I> and modifies a list of <I>*.in</I> files. For <I>config.cache,</I> and modifies a list of <I>*.in</I> files. For
example, if there exists a <I>Makefile.in,</I> configure generates a example, if there exists a <I>Makefile.in,</I> configure generates
<I>Makefile</I> that contains substitutions for all @var@ parameters a <I>Makefile</I> that contains substitutions for all @var@
found by configure.<P> parameters found by configure.</P>
When you need to edit files, make sure you don't waste time modifying <P>When you need to edit files, make sure you don't waste time
files generated by <I>configure.</I> Edit the <I>*.in</I> file, and modifying files generated by <I>configure.</I> Edit the <I>*.in</I>
re-run <I>configure</I> to recreate the needed file. If you run <I>make file, and re-run <I>configure</I> to recreate the needed file. If
distclean</I> from the top-level source directory, all files derived by you run <I>make distclean</I> from the top-level source directory,
configure are removed, so you see only the file contained in the source all files derived by configure are removed, so you see only the
distribution.<P> file contained in the source distribution.</P>
<H3><a name="12">12</a>) How do I add a new port?</H3><P> <H3><A name="12">12</A>) How do I add a new port?</H3>
There are a variety of places that need to be modified to add a new <P>There are a variety of places that need to be modified to add a
port. First, start in the <I>src/template</I> directory. Add an new port. First, start in the <I>src/template</I> directory. Add an
appropriate entry for your OS. Also, use <I>src/config.guess</I> to add appropriate entry for your OS. Also, use <I>src/config.guess</I> to
your OS to <I>src/template/.similar.</I> You shouldn't match the OS add your OS to <I>src/template/.similar.</I> You shouldn't match
version exactly. The <I>configure</I> test will look for an exact OS the OS version exactly. The <I>configure</I> test will look for an
version number, and if not found, find a match without version number. exact OS version number, and if not found, find a match without
Edit <I>src/configure.in</I> to add your new OS. (See configure item version number. Edit <I>src/configure.in</I> to add your new OS.
above.) You will need to run autoconf, or patch <I>src/configure</I> (See configure item above.) You will need to run autoconf, or patch
too.<P> <I>src/configure</I> too.</P>
Then, check <I>src/include/port</I> and add your new OS file, with <P>Then, check <I>src/include/port</I> and add your new OS file,
appropriate values. Hopefully, there is already locking code in with appropriate values. Hopefully, there is already locking code
<I>src/include/storage/s_lock.h</I> for your CPU. There is also a in <I>src/include/storage/s_lock.h</I> for your CPU. There is also
<I>src/makefiles</I> directory for port-specific Makefile handling. a <I>src/makefiles</I> directory for port-specific Makefile
There is a <I>backend/port</I> directory if you need special files for handling. There is a <I>backend/port</I> directory if you need
your OS.<P> special files for your OS.</P>
<H3><a name="13">13</a>) What is CommandCounterIncrement()?</H3><P> <H3><A name="13">13</A>) What is CommandCounterIncrement()?</H3>
Normally, transactions can not see the rows they modify. This allows <CODE> <P>Normally, transactions can not see the rows they modify. This
UPDATE foo SET x = x + 1</CODE> to work correctly. allows <CODE>UPDATE foo SET x = x + 1</CODE> to work correctly.</P>
<P>
<P>However, there are cases where a transactions needs to see rows
However, there are cases where a transactions needs to see rows affected affected in previous parts of the transaction. This is accomplished
in previous parts of the transaction. This is accomplished using a using a Command Counter. Incrementing the counter allows
Command Counter. Incrementing the counter allows transactions to be transactions to be broken into pieces so each piece can see rows
broken into pieces so each piece can see rows modified by previous modified by previous pieces. <I>CommandCounterIncrement()</I>
pieces. <I>CommandCounterIncrement()</I> increments the Command increments the Command Counter, creating a new part of the
Counter, creating a new part of the transaction. <P> transaction.</P>
<H3><A name="14">14</A>) Why don't we use threads in the
backend?</H3>
<P>There are several reasons threads are not used:</P>
<UL>
<LI>Historically, threads were unsupported and buggy.</LI>
<LI>An error in one backend can corrupt other backends.</LI>
<LI>Speed improvements using threads are small compared to the
remaining backend startup time.</LI>
<LI>The backend code would be more complex.</LI>
</UL>
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