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<HTML>
<HEAD>
<TITLE>PostgreSQL Developers FAQ</title>
</HEAD>
<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#FF0000" VLINK="#A00000" ALINK="#0000FF">
<H1>
Developer's Frequently Asked Questions (FAQ) for PostgreSQL
</H1>
<P>
Last updated: Fri Dec 24 11:43:42 EST 1999
<P>
Current maintainer: Bruce Momjian (<a
href="mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</a>)<BR>
<P>
The most recent version of this document can be viewed at
the postgreSQL Web site, <a
href="http://PostgreSQL.org">http://PostgreSQL.org</a>.
<P>
<HR>
<P>
<CENTER><H2>Questions</H2></CENTER>
<a href="#1">1</a>) What tools are available for developers?<BR>
<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>
<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="#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>
<a href="#8">8</a>) Why are table, column, type, function, view
names sometimes referenced as <I>Name</I> or <I>NameData,</I> and
sometimes as <I>char *?</I><BR>
<a href="#9">9</a>) How do I efficiently access information in
tables from the backend code?<BR>
<a href="#10">10</a>) What is elog()?<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>
<BR>
<HR>
<H3><a
name="1">1</a>) What tools are available for developers?</H3><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.
<PRE>
RELEASE_CHANGES changes we have to make for each release
SQL_keywords standard SQL'92 keywords
backend description/flowchart of the backend directories
ccsym find standard defines made by your compiler
entab converts tabs to spaces, used by pgindent
find_static finds functions that could be made static
find_typedef get a list of typedefs in the source code
make_ctags make vi 'tags' file in each directory
make_diff make *.orig and diffs of source
make_etags make emacs 'etags' files
make_keywords.README make comparison of our keywords and SQL'92
make_mkid make mkid ID files
mkldexport create AIX exports file
pgindent indents C source files
pginclude scripts for adding/removing include files
unused_oids in pgsql/src/include/catalog
</PRE>
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,
you will see few paragraphs describing the data flow, the backend
components in a flow chart, and a description of the shared memory area.
You can click on any flowchart box to see a description. If you then
click on the directory name, you will be taken to the source directory,
to browse the actual source code behind it. We also have several README
files in some source directories to describe the function of the module.
The browser will display these when you enter the directory also. The
<I>tools/backend</I> directory is also contained on our web page under
the title <I>How PostgreSQL Processes a Query.</I><P>
Second, you really should have an editor that can handle tags, so you
can tag a function call to see the function definition, and then tag
inside that function to see an even lower-level function, and then back
out twice to return to the original function. Most editors support this
via <I>tags</I> or <I>etags</I> files.<P>
Third, you need to get <I>mkid</I> from ftp.postgresql.org. By running
<I>tools/make_mkid</I>, an archive of source symbols can be created that
can be rapidly queried like <I>grep</I> or edited. Others prefer
<I>glimpse.</I><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
each tab is four spaces. You will need to set your editor to display
tabs as four spaces:
<BR>
<PRE>
vi in ~/.exrc:
set tabstop=4
set sw=4
more:
more -x4
less:
less -x4
emacs:
M-x set-variable tab-width
or
; Cmd to set tab stops &etc for working with PostgreSQL code
(defun pgsql-mode ()
"Set PostgreSQL C indenting conventions in current buffer."
(interactive)
(c-mode) ; necessary to make c-set-offset local!
(setq tab-width 4) ; already buffer-local
; (setq comment-column 48) ; already buffer-local
(c-set-style "bsd")
(c-set-offset 'case-label '+)
)
and add this to your autoload list (modify file path in macro):
(setq auto-mode-alist
(cons '("\\`/usr/local/src/pgsql/.*\\.[chyl]\\'" . pgsql-c-mode)
auto-mode-alist))
or
/*
* Local variables:
* tab-width: 4
* c-indent-level: 4
* c-basic-offset: 4
* End:
*/
</PRE>
<BR>
<I>pgindent</I> will the format code by specifying
flags to your operating system's utility <I>indent.</I><P>
<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.
Comment blocks that need specific line breaks should be formatted as
<I>block comments,</I> where the comment starts as
<CODE>/*------</CODE>. These comments will not be reformatted in any
way.
<I>pginclude</I> contains scripts used to add needed #include's to
include files, and removed unneeded #include's.
When adding system types, you will need to assign oids to them.
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><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 Standard,</I> by C.J.
Date, et. al, Addison, Wesley, <I>Fundamentals of Database Systems,</I>
by Elmasri and Navathe, and <I>Transaction Processing,</I> by Jim Gray,
Morgan, Kaufmann<P>
There is also a database performance site, with a handbook on-line
written by Jim Gray at <A
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>
Here are some of the <I>List</I> manipulation commands:
<BLOCKQUOTE>
<DL>
<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>
<CODE>
List *i, *list;
foreach(i, list)
{
Var *var = lfirst(i);
/* process var here */
}
</CODE>
</PRE>
<DT>lcons(node, list)
<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>lappend(list, node)
<DD>add <I>node</I> to the end of <I>list.</I> This is more expensive
that lcons.
<DT>nconc(list1, list2)
<DD>Concat <I>list2</I> on to the end of <I>list1.</I>
<DT>length(list)
<DD>return the length of the <I>list.</I>
<DT>nth(i, list)
<DD>return the <I>i</I>'th element in <I>list.</I>
<DT>lconsi, ...
<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.
</DL>
</BLOCKQUOTE>
You can print nodes easily inside <I>gdb.</I> First, to disable
output truncation when you use the gdb <I>print</I> command:
<PRE>
<CODE>
(gdb) set print elements 0
</CODE>
</PRE>
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
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 the second in a long format:
<PRE>
<CODE>
(gdb) call print(any_pointer)
(gdb) call pprint(any_pointer)
</CODE>
</PRE>
The output appears in the postmaster log file, or on your screen if 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
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
added with surprisingly little code. I often start by adding 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>
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
tree?</H3><P>
There are several ways to obtain the source tree. Occasional developers
can just get the most recent source tree snapshot from
ftp.postgresql.org. For regular developers, you can use CVS. CVS
allows you to download the source tree, then occasionally update your
copy of the source tree with any new changes. Using CVS, you don't have
to download the entire source each time, only the changed files.
Anonymous CVS does not allows developers to update the remote source
tree, though privileged developers can do this. There is a CVS FAQ on
our web site that describes how to use remote CVS. You can also use
CVSup, which has similarly functionality, and is available from
ftp.postgresql.org.<P>
To update the source tree, there are two ways. You can generate a patch
against your current source tree, perhaps using the make_diff tools
mentioned above, and send them to the patches list. They will be
reviewed, and applied in a timely manner. If the patch is major, and we
are in beta testing, the developers may wait for the final release
before applying your patches.<P>
For hard-core developers, Marc(scrappy@postgresql.org) will give you a
Unix shell account on postgresql.org, so you can use CVS to update the
main source tree, or you can ftp your files into your 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>
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
<I>src/test/regress/checkresults</I> with and without your changes, to
see that your patch does not change the regression test in unexpected
ways. This practice has saved me many times. The regression tests test
the code in ways I would never do, and has caught many bugs in my
patches. By finding the problems now, you 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
should I do?</H3><P>
The structures passing around from the parser, rewrite, optimizer, and
executor require quite a bit of support. Most structures have support
routines in <I>src/backend/nodes</I> used to create, copy, read, and output
those structures. Make sure you add support for your new field to these
files. Find any other 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
names sometimes referenced as <I>Name</I> or <I>NameData,</I> and
sometimes as <I>char *?</I></H3><P>
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>
typedef struct nameData
{
char data[NAMEDATALEN];
} NameData;
typedef NameData *Name;
</CODE></PRE>
Table, column, type, function, and view names that come into the
backend via user queries are stored as variable-length, null-terminated
character strings.<P>
Many functions are called with both types of names, ie. <I>heap_open().</I>
Because the Name type is null-terminated, it is safe to pass it to a
function expecting a char *. Because there are many cases where on-disk
names(Name) are compared to user-supplied 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>
You first need to find the tuples(rows) you are interested in. There
are two ways. First, <I>SearchSysCacheTuple()</I> and related functions
allow you to query the system catalogs. This is the preferred way to
access system tables, because the first call to the cache loads the
needed rows, and future requests can return the results without
accessing the base table. The caches use system table indexes
to look up tuples. A list of available caches is located in
<I>src/backend/utils/cache/syscache.c.</I>
<I>src/backend/utils/cache/lsyscache.c</I> contains many column-specific
cache lookup functions.<P>
The rows returned are cached-owned versions of the heap rows. They are
invalidated when the base table changes. Because the cache is local to
each backend, you may use the pointer returned from the cache for short
periods without making a copy of the tuple. If you send the pointer
into a large function that will be doing its own cache lookups, it is
possible the cache entry may be flushed, so you should use
<I>SearchSysCacheTupleCopy()</I> in these cases, and <I>pfree()</I> the
tuple when you are done.<P>
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
all backends. The backend automatically takes care of loading the rows
into the buffer cache.<P>
Open the table with <I>heap_open().</I> You can then start a table scan
with <I>heap_beginscan(),</I> then use <I>heap_getnext()</I> and
continue as long as <I>HeapTupleIsValid()</I> returns true. Then do a
<I>heap_endscan().</I> <I>Keys</I> can be assigned to the <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>
You can also use <I>heap_fetch()</I> to fetch rows by block
number/offset. While scans automatically lock/unlock rows from the
buffer cache, with <I>heap_fetch(),</I> you must pass a <I>Buffer</I>
pointer, and <I>ReleaseBuffer()</I> it when completed.
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
<I>HeapTuple</I> structure entries.
If you need a table-specific column, you should take the HeapTuple
pointer, and use the <I>GETSTRUCT()</I> macro to access the
table-specific start of the tuple. You then cast the pointer as a
<I>Form_pg_proc</I> pointer if you are accessing the pg_proc table, or
<I>Form_pg_type</I> if you are accessing pg_type. You can then access
the columns by using a structure pointer:
<PRE>
<CODE>
((Form_pg_class) GETSTRUCT(tuple))-&gt;relnatts
</CODE>
</PRE>
You should not directly change <I>live</I> tuples in this way. The best
way is to use <I>heap_tuplemodify()</I> and pass it your palloc'ed
tuple, and the values you want changed. It returns another palloc'ed
tuple, which you pass to <I>heap_replace().</I>
You can delete tuples by passing the tuple's <I>t_self</I> to
<I>heap_destroy().</I> You can use it for <I>heap_update()</I> too.
Remember, tuples can be either system cache versions, which may go away
soon after you get them, buffer cache versions, which 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
palloc'ed tuple, that you must <I>pfree()</I> when finished.
<H3><a name="10">10</a>) What is elog()?</H3><P>
<I>elog()</I> is used to send messages to the front-end, and optionally
terminate the current query being processed. The first parameter is an
elog level of <I>NOTICE,</I> <I>DEBUG,</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 postmaster logs. <I>ERROR</I> prints in
both places, and terminates the current query, never returning from the call.
<I>FATAL</I> terminates the backend process.
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><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 various
capabilities of the OS, and to set variables that can then be tested in
C programs and Makefiles. Autoconf is installed on the PostgreSQL main
server. To add options to configure, edit <I>configure.in,</I> and then
run <I>autoconf</I> to generate <I>configure.</I><P>
When <I>configure</I> is run by the user, it tests various OS
capabilities, stores those in <I>config.status</I> and
<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
<I>Makefile</I> that contains substitutions for all @var@ parameters
found by configure.<P>
When you need to edit files, make sure you don't waste time modifying
files generated by <I>configure.</I> Edit the <I>*.in</I> file, and
re-run <I>configure</I> to recreate the needed file. If you run <I>make
distclean</I> from the top-level source directory, all files derived by
configure are removed, so you see only the file contained in the source
distribution.<P>
<H3><a name="12">12</a>) How do I add a new port?</H3><P>
There are a variety of places that need to be modified to add a 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
your OS to <I>src/template/.similar.</I> You shouldn't match the OS
version exactly. The <I>configure</I> test will look for an exact OS
version number, and if not found, find a match without version number.
Edit <I>src/configure.in</I> to add your new OS. (See configure item
above.) You will need to run autoconf, or patch <I>src/configure</I>
too.<P>
Then, check <I>src/include/port</I> and add your new OS file, with
appropriate values. Hopefully, there is already locking code in
<I>src/include/storage/s_lock.h</I> for your CPU. There is also a
<I>src/makefiles</I> directory for port-specific Makefile handling.
There is a <I>backend/port</I> directory if you need special files for
your OS.<P>
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