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@ -9,31 +9,146 @@
postgreSQL Web site, http://www.PostgreSQL.org.
_________________________________________________________________
Questions
General Questions
1) What tools are available for developers?
2) What books are good for developers?
3) Why do we use palloc() and pfree() to allocate memory?
4) Why do we use Node and List to make data structures?
5) How do I add a feature or fix a bug?
6) How do I download/update the current source tree?
7) How do I test my changes?
7) I just added a field to a structure. What else should I do?
8) Why are table, column, type, function, view names sometimes
1.1) How do I get involved in PostgreSQL development?
1.2) How do I add a feature or fix a bug?
1.3) How do I download/update the current source tree?
1.4) How do I test my changes?
1.5) What tools are available for developers?
1.6) What books are good for developers?
1.7) What is configure all about?
1.8) How do I add a new port?
1.9) Why don't we use threads in the backend?
1.10) How are RPM's packaged?
1.11) How are CVS branches handled?
Technical Questions
2.1) How do I efficiently access information in tables from the
backend code?
2.2) Why are table, column, type, function, view names sometimes
referenced as Name or NameData, and sometimes as char *?
9) How do I efficiently access information in tables from the backend
code?
10) What is elog()?
11) What is configure all about?
12) How do I add a new port?
13) What is CommandCounterIncrement()?
14) Why don't we use threads in the backend?
15) How are RPM's packaged?
16) How are CVS branches handled?
17) How do I get involved in PostgreSQL development?
2.3) Why do we use Node and List to make data structures?
2.4) I just added a field to a structure. What else should I do?
2.5) Why do we use palloc() and pfree() to allocate memory?
2.6) What is elog()?
2.7) What is CommandCounterIncrement()?
_________________________________________________________________
1) What tools are available for developers?
General Questions
1.1) How go I get involved in PostgreSQL development?
This was written by Lamar Owen:
2001-06-22
What open source development process is used by the PostgreSQL team?
Read HACKERS for six months (or a full release cycle, whichever is
longer). Really. HACKERS _is_the process. The process is not well
documented (AFAIK -- it may be somewhere that I am not aware of) --
and it changes continually.
What development environment (OS, system, compilers, etc) is required
to develop code?
Developers Corner on the website has links to this information. The
distribution tarball itself includes all the extra tools and documents
that go beyond a good Unix-like development environment. In general, a
modern unix with a modern gcc, GNU make or equivalent, autoconf (of a
particular version), and good working knowledge of those tools are
required.
What areas need support?
The TODO list.
You've made the first step, by finding and subscribing to HACKERS.
Once you find an area to look at in the TODO, and have read the
documentation on the internals, etc, then you check out a current
CVS,write what you are going to write (keeping your CVS checkout up to
date in the process), and make up a patch (as a context diff only) and
send to the PATCHES list, prefereably.
Discussion on the patch typically happens here. If the patch adds a
major feature, it would be a good idea to talk about it first on the
HACKERS list, in order to increase the chances of it being accepted,
as well as toavoid duplication of effort. Note that experienced
developers with a proven track record usually get the big jobs -- for
more than one reason. Also note that PostgreSQL is highly portable --
nonportable code will likely be dismissed out of hand.
Once your contributions get accepted, things move from there.
Typically, you would be added as a developer on the list on the
website when one of the other developers recommends it. Membership on
the steering committee is by invitation only, by the other steering
committee members, from what I have gathered watching froma distance.
I make these statements from having watched the process for over two
years.
To see a good example of how one goes about this, search the archives
for the name 'Tom Lane' and see what his first post consisted of, and
where he took things. In particular, note that this hasn't been _that_
long ago -- and his bugfixing and general deep knowledge with this
codebase is legendary. Take a few days to read after him. And pay
special attention to both the sheer quantity as well as the
painstaking quality of his work. Both are in high demand.
1.2) How do I add a feature or fix a bug?
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.
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.
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.
1.3) How do I download/update the current source tree?
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.
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.
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.
1.4) How do I test my changes?
First, use psql to make sure it is working as you expect. Then run
src/test/regress and get the output of src/test/regress/checkresults
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.
1.5) What tools are available for developers?
Aside from the User documentation mentioned in the regular FAQ, there
are several development tools available. First, all the files in the
@ -126,264 +241,32 @@
*/
pgindent will the format code by specifying flags to your operating
system's utility indent.
pgindent 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 block
comments, where the comment starts as /*------. These comments will
not be reformatted in any way.
pginclude 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 unused_oids in pgsql/src/include/catalog that
shows the unused oids.
2) What books are good for developers?
I have four good books, An Introduction to Database Systems, by C.J.
Date, Addison, Wesley, A Guide to the SQL Standard, by C.J. Date, et.
al, Addison, Wesley, Fundamentals of Database Systems, by Elmasri and
Navathe, and Transaction Processing, by Jim Gray, Morgan, Kaufmann
There is also a database performance site, with a handbook on-line
written by Jim Gray at http://www.benchmarkresources.com.
3) Why do we use palloc() and pfree() to allocate memory?
palloc() and pfree() 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.
4) Why do we use Node and List to make data structures?
We do this because this allows a consistent way to pass data inside
the backend in a flexible way. Every node has a NodeTag which
specifies what type of data is inside the Node. Lists are groups of
Nodes chained together as a forward-linked list.
Here are some of the List manipulation commands:
lfirst(i)
return the data at list element i.
lnext(i)
return the next list element after i.
foreach(i, list)
loop through list, assigning each list element to i. It is
important to note that i is a List *, not the data in the List
element. You need to use lfirst(i) to get at the data. Here is
a typical code snipped that loops through a List containing Var
*'s and processes each one:
List *i, *list;
foreach(i, list)
{
Var *var = lfirst(i);
/* process var here */
}
lcons(node, list)
add node to the front of list, or create a new list with node
if list is NIL.
lappend(list, node)
add node to the end of list. This is more expensive that lcons.
nconc(list1, list2)
Concat list2 on to the end of list1.
length(list)
return the length of the list.
nth(i, list)
return the i'th element in list.
lconsi, ...
There are integer versions of these: lconsi, lappendi, nthi.
List's containing integers instead of Node pointers are used to
hold list of relation object id's and other integer quantities.
You can print nodes easily inside gdb. First, to disable output
truncation when you use the gdb print command:
(gdb) set print elements 0
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:
(gdb) call print(any_pointer)
(gdb) call pprint(any_pointer)
The output appears in the postmaster log file, or on your screen if
you are running a backend directly without a postmaster.
5) How do I add a feature or fix a bug?
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.
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.
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.
6) How do I download/update the current source tree?
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.
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.
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.
6) How do I test my changes?
First, use psql to make sure it is working as you expect. Then run
src/test/regress and get the output of src/test/regress/checkresults
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.
7) I just added a field to a structure. What else should I do?
The structures passing around from the parser, rewrite, optimizer, and
executor require quite a bit of support. Most structures have support
routines in src/backend/nodes 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. mkid is helpful with this (see above).
8) Why are table, column, type, function, view names sometimes referenced as
Name or NameData, and sometimes as char *?
Table, column, type, function, and view names are stored in system
tables in columns of type Name. Name is a fixed-length,
null-terminated type of NAMEDATALEN bytes. (The default value for
NAMEDATALEN is 32 bytes.)
typedef struct nameData
{
char data[NAMEDATALEN];
} NameData;
typedef NameData *Name;
Table, column, type, function, and view names that come into the
backend via user queries are stored as variable-length,
null-terminated character strings.
Many functions are called with both types of names, ie. heap_open().
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.
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
are two ways. First, SearchSysCache() 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
src/backend/utils/cache/syscache.c.
src/backend/utils/cache/lsyscache.c contains many column-specific
cache lookup functions.
The rows returned are cache-owned versions of the heap rows.
Therefore, you must not modify or delete the tuple returned by
SearchSysCache(). What you should do is release it with
ReleaseSysCache() when you are done using it; this informs the cache
that it can discard that tuple if necessary. If you neglect to call
ReleaseSysCache(), then the cache entry will remain locked in the
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
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.
Open the table with heap_open(). You can then start a table scan with
heap_beginscan(), then use heap_getnext() and continue as long as
HeapTupleIsValid() returns true. Then do a heap_endscan(). Keys can be
assigned to the scan. No indexes are used, so all rows are going to be
compared to the keys, and only the valid rows returned.
You can also use heap_fetch() to fetch rows by block number/offset.
While scans automatically lock/unlock rows from the buffer cache, with
heap_fetch(), you must pass a Buffer pointer, and ReleaseBuffer() it
when completed.
Once you have the row, you can get data that is common to all tuples,
like t_self and t_oid, by merely accessing the HeapTuple structure
entries. If you need a table-specific column, you should take the
HeapTuple pointer, and use the GETSTRUCT() macro to access the
table-specific start of the tuple. You then cast the pointer as a
Form_pg_proc pointer if you are accessing the pg_proc table, or
Form_pg_type if you are accessing pg_type. You can then access the
columns by using a structure pointer:
((Form_pg_class) GETSTRUCT(tuple))->relnatts
You must not directly change live tuples in this way. The best way is
to use heap_modifytuple() and pass it your original tuple, and the
values you want changed. It returns a palloc'ed tuple, which you pass
to heap_replace(). You can delete tuples by passing the tuple's t_self
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
call ReleaseSysCache(), or read directly from disk buffers, which go
away when you heap_getnext(), heap_endscan, or ReleaseBuffer(), in the
heap_fetch() case. Or it may be a palloc'ed tuple, that you must
pfree() when finished.
system's utility indent.
pgindent 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 block
comments, where the comment starts as /*------. These comments will
not be reformatted in any way.
pginclude 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 unused_oids in pgsql/src/include/catalog that
shows the unused oids.
10) What is elog()?
1.6) What books are good for developers?
elog() 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 NOTICE, DEBUG, ERROR, or FATAL. NOTICE prints on the
user's terminal and the postmaster logs. DEBUG prints only in the
postmaster logs. ERROR prints in both places, and terminates the
current query, never returning from the call. FATAL terminates the
backend process. The remaining parameters of elog are a printf-style
set of parameters to print.
I have four good books, An Introduction to Database Systems, by C.J.
Date, Addison, Wesley, A Guide to the SQL Standard, by C.J. Date, et.
al, Addison, Wesley, Fundamentals of Database Systems, by Elmasri and
Navathe, and Transaction Processing, by Jim Gray, Morgan, Kaufmann
There is also a database performance site, with a handbook on-line
written by Jim Gray at http://www.benchmarkresources.com.
11) What is configure all about?
1.7) What is configure all about?
The files configure and configure.in are part of the GNU autoconf
package. Configure allows us to test for various capabilities of the
@ -405,7 +288,7 @@ typedef struct nameData
removed, so you see only the file contained in the source
distribution.
12) How do I add a new port?
1.8) How do I add a new port?
There are a variety of places that need to be modified to add a new
port. First, start in the src/template directory. Add an appropriate
@ -422,19 +305,7 @@ typedef struct nameData
src/makefiles directory for port-specific Makefile handling. There is
a backend/port directory if you need special files for your OS.
13) What is CommandCounterIncrement()?
Normally, transactions can not see the rows they modify. This allows
UPDATE foo SET x = x + 1 to work correctly.
However, there are cases where a transactions needs to see rows
affected in previous parts of the transaction. This is accomplished
using a Command Counter. Incrementing the counter allows transactions
to be broken into pieces so each piece can see rows modified by
previous pieces. CommandCounterIncrement() increments the Command
Counter, creating a new part of the transaction.
14) Why don't we use threads in the backend?
1.9) Why don't we use threads in the backend?
There are several reasons threads are not used:
* Historically, threads were unsupported and buggy.
@ -443,7 +314,7 @@ typedef struct nameData
remaining backend startup time.
* The backend code would be more complex.
15) How are RPM's packaged?
1.10) How are RPM's packaged?
This was written by Lamar Owen:
@ -538,7 +409,7 @@ typedef struct nameData
Of course, there are many projects that DO include all the files
necessary to build RPMs from their Official Tarball (TM).
16) How are CVS branches managed?
1.11) How are CVS branches managed?
This was written by Tom Lane:
@ -597,58 +468,194 @@ typedef struct nameData
tree right away after a major release --- we wait for a dot-release or
two, so that we won't have to double-patch the first wave of fixes.
17) How go I get involved in PostgreSQL development?
Technical Questions
2.1) How do I efficiently access information in tables from the backend code?
This was written by Lamar Owen:
You first need to find the tuples(rows) you are interested in. There
are two ways. First, SearchSysCache() 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
src/backend/utils/cache/syscache.c.
src/backend/utils/cache/lsyscache.c contains many column-specific
cache lookup functions.
2001-06-22
What open source development process is used by the PostgreSQL team?
The rows returned are cache-owned versions of the heap rows.
Therefore, you must not modify or delete the tuple returned by
SearchSysCache(). What you should do is release it with
ReleaseSysCache() when you are done using it; this informs the cache
that it can discard that tuple if necessary. If you neglect to call
ReleaseSysCache(), then the cache entry will remain locked in the
cache until end of transaction, which is tolerable but not very
desirable.
Read HACKERS for six months (or a full release cycle, whichever is
longer). Really. HACKERS _is_the process. The process is not well
documented (AFAIK -- it may be somewhere that I am not aware of) --
and it changes continually.
What development environment (OS, system, compilers, etc) is required
to develop code?
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.
Developers Corner on the website has links to this information. The
distribution tarball itself includes all the extra tools and documents
that go beyond a good Unix-like development environment. In general, a
modern unix with a modern gcc, GNU make or equivalent, autoconf (of a
particular version), and good working knowledge of those tools are
required.
What areas need support?
Open the table with heap_open(). You can then start a table scan with
heap_beginscan(), then use heap_getnext() and continue as long as
HeapTupleIsValid() returns true. Then do a heap_endscan(). Keys can be
assigned to the scan. No indexes are used, so all rows are going to be
compared to the keys, and only the valid rows returned.
The TODO list.
You can also use heap_fetch() to fetch rows by block number/offset.
While scans automatically lock/unlock rows from the buffer cache, with
heap_fetch(), you must pass a Buffer pointer, and ReleaseBuffer() it
when completed.
You've made the first step, by finding and subscribing to HACKERS.
Once you find an area to look at in the TODO, and have read the
documentation on the internals, etc, then you check out a current
CVS,write what you are going to write (keeping your CVS checkout up to
date in the process), and make up a patch (as a context diff only) and
send to the PATCHES list, prefereably.
Once you have the row, you can get data that is common to all tuples,
like t_self and t_oid, by merely accessing the HeapTuple structure
entries. If you need a table-specific column, you should take the
HeapTuple pointer, and use the GETSTRUCT() macro to access the
table-specific start of the tuple. You then cast the pointer as a
Form_pg_proc pointer if you are accessing the pg_proc table, or
Form_pg_type if you are accessing pg_type. You can then access the
columns by using a structure pointer:
((Form_pg_class) GETSTRUCT(tuple))->relnatts
You must not directly change live tuples in this way. The best way is
to use heap_modifytuple() and pass it your original tuple, and the
values you want changed. It returns a palloc'ed tuple, which you pass
to heap_replace(). You can delete tuples by passing the tuple's t_self
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
call ReleaseSysCache(), or read directly from disk buffers, which go
away when you heap_getnext(), heap_endscan, or ReleaseBuffer(), in the
heap_fetch() case. Or it may be a palloc'ed tuple, that you must
pfree() when finished.
Discussion on the patch typically happens here. If the patch adds a
major feature, it would be a good idea to talk about it first on the
HACKERS list, in order to increase the chances of it being accepted,
as well as toavoid duplication of effort. Note that experienced
developers with a proven track record usually get the big jobs -- for
more than one reason. Also note that PostgreSQL is highly portable --
nonportable code will likely be dismissed out of hand.
2.2) Why are table, column, type, function, view names sometimes referenced
as Name or NameData, and sometimes as char *?
Table, column, type, function, and view names are stored in system
tables in columns of type Name. Name is a fixed-length,
null-terminated type of NAMEDATALEN bytes. (The default value for
NAMEDATALEN is 32 bytes.)
typedef struct nameData
{
char data[NAMEDATALEN];
} NameData;
typedef NameData *Name;
Table, column, type, function, and view names that come into the
backend via user queries are stored as variable-length,
null-terminated character strings.
Once your contributions get accepted, things move from there.
Typically, you would be added as a developer on the list on the
website when one of the other developers recommends it. Membership on
the steering committee is by invitation only, by the other steering
committee members, from what I have gathered watching froma distance.
Many functions are called with both types of names, ie. heap_open().
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.
I make these statements from having watched the process for over two
years.
2.3) Why do we use Node and List to make data structures?
We do this because this allows a consistent way to pass data inside
the backend in a flexible way. Every node has a NodeTag which
specifies what type of data is inside the Node. Lists are groups of
Nodes chained together as a forward-linked list.
To see a good example of how one goes about this, search the archives
for the name 'Tom Lane' and see what his first post consisted of, and
where he took things. In particular, note that this hasn't been _that_
long ago -- and his bugfixing and general deep knowledge with this
codebase is legendary. Take a few days to read after him. And pay
special attention to both the sheer quantity as well as the
painstaking quality of his work. Both are in high demand.
Here are some of the List manipulation commands:
lfirst(i)
return the data at list element i.
lnext(i)
return the next list element after i.
foreach(i, list)
loop through list, assigning each list element to i. It is
important to note that i is a List *, not the data in the List
element. You need to use lfirst(i) to get at the data. Here is
a typical code snipped that loops through a List containing Var
*'s and processes each one:
List *i, *list;
foreach(i, list)
{
Var *var = lfirst(i);
/* process var here */
}
lcons(node, list)
add node to the front of list, or create a new list with node
if list is NIL.
lappend(list, node)
add node to the end of list. This is more expensive that lcons.
nconc(list1, list2)
Concat list2 on to the end of list1.
length(list)
return the length of the list.
nth(i, list)
return the i'th element in list.
lconsi, ...
There are integer versions of these: lconsi, lappendi, nthi.
List's containing integers instead of Node pointers are used to
hold list of relation object id's and other integer quantities.
You can print nodes easily inside gdb. First, to disable output
truncation when you use the gdb print command:
(gdb) set print elements 0
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:
(gdb) call print(any_pointer)
(gdb) call pprint(any_pointer)
The output appears in the postmaster log file, or on your screen if
you are running a backend directly without a postmaster.
2.4) I just added a field to a structure. What else should I do?
The structures passing around from the parser, rewrite, optimizer, and
executor require quite a bit of support. Most structures have support
routines in src/backend/nodes 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. mkid is helpful with this (see above).
2.5) Why do we use palloc() and pfree() to allocate memory?
palloc() and pfree() 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.
2.6) What is elog()?
elog() 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 NOTICE, DEBUG, ERROR, or FATAL. NOTICE prints on the
user's terminal and the postmaster logs. DEBUG prints only in the
postmaster logs. ERROR prints in both places, and terminates the
current query, never returning from the call. FATAL terminates the
backend process. The remaining parameters of elog are a printf-style
set of parameters to print.
2.7) What is CommandCounterIncrement()?
Normally, transactions can not see the rows they modify. This allows
UPDATE foo SET x = x + 1 to work correctly.
However, there are cases where a transactions needs to see rows
affected in previous parts of the transaction. This is accomplished
using a Command Counter. Incrementing the counter allows transactions
to be broken into pieces so each piece can see rows modified by
previous pieces. CommandCounterIncrement() increments the Command
Counter, creating a new part of the transaction.

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