
          Developer's Frequently Asked Questions (FAQ) for PostgreSQL
                                       
   Last updated: Mon Feb 22 17:15:06 EST 1999
   
   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.
     _________________________________________________________________
   
                                 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()?
     _________________________________________________________________
   
  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. Others prefer glimpse.
   
   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 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, 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_self 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 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 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_self 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.
