Developer's Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Fri Oct 2 15:21:32 EDT 1998
Current maintainer: Bruce Momjian (maillist@candle.pha.pa.us)
The most recent version of this document can be viewed at the
postgreSQL Web site, http://postgreSQL.org.
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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
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()?
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1) 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
/tools directory are designed for developers.
RELEASE_CHANGES changes we have to make for each release
SQL_keywords standard SQL'92 keywords
backend description/flowchart of the backend directorie
s
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
Let me note some of these. If you point your browser at the
file:/usr/local/src/pgsql/src/tools/backend/index.html 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 tools/backend directory is also contained on
our web page under the title How PostgreSQL Processes a Query.
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 tags or etags files.
Third, you need to get mkid from ftp.postgresql.org. By running
tools/make_mkid, an archive of source symbols can be created that can
be rapidly queried like grep or edited.
make_diff has tools to create patch diff files that can be applied to
the distribution.
pgindent will format source files to match our standard format, which
has four-space tabs, and an indenting format specified by flags to the
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.
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 lists of
Nodes. lfirst(), lnext(), and foreach() are used to get, skip, and
traverse through Lists.
You can print nodes easily inside gdb. First, to disable output
truncation:
(gdb) set print elements 0
You may then use either of the next two commands to print out List,
Node, and structure contents. The first prints in a short format, and
the second in a long format:
(gdb) call print(any_pointer)
(gdb) call pprint(any_pointer)
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 in to 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, SearchSysCacheTuple() 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. Some of 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 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 SearchSysCacheTupleCopy() in these cases, and pfree() the tuple
when you are done.
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_ctid and t_oid, by mererly 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 TypeTupleForm if you are accessing pg_type. You can then
access the columns by using a structure pointer:
((Form_pg_class) GETSTRUCT(tuple))->relnatts
You should not directly change live tuples in this way. The best way
is to use heap_tuplemodify() and pass it your palloc'ed tuple, and the
values you want changed. It returns another palloc'ed tuple, which you
pass to heap_replace(). You can delete tuples by passing the tuple's
t_ctid to heap_destroy(). Remember, tuples can be either system cache
versions, which may go away soon after you get them, buffer cache
version, which will 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.
10) 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.