Skip to content
Snippets Groups Projects
Select Git revision
  • benchmark-tools
  • postgres-lambda
  • master default
  • REL9_4_25
  • REL9_5_20
  • REL9_6_16
  • REL_10_11
  • REL_11_6
  • REL_12_1
  • REL_12_0
  • REL_12_RC1
  • REL_12_BETA4
  • REL9_4_24
  • REL9_5_19
  • REL9_6_15
  • REL_10_10
  • REL_11_5
  • REL_12_BETA3
  • REL9_4_23
  • REL9_5_18
  • REL9_6_14
  • REL_10_9
  • REL_11_4
23 results

FAQ_DEV

Blame
  • FAQ_DEV 19.98 KiB
    
              Developer's Frequently Asked Questions (FAQ) for PostgreSQL
                                           
       Last updated: Fri Jun 9 21:54:54 EDT 2000
       
       Current maintainer: Bruce Momjian (pgman@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()?
       11) What is configure all about?
       12) How do I add a new port?
       13) What is CommandCounterIncrement()?
         _________________________________________________________________
       
      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
            pginclude               scripts for adding/removing include files
            unused_oids             in pgsql/src/include/catalog
    
       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 id-utils from:
            ftp://alpha.gnu.org/gnu/id-utils-3.2d.tar.gz
            ftp://tug.org/gnu/id-utils-3.2d.tar.gz
            ftp://ftp.enst.fr/pub/gnu/gnits/id-utils-3.2d.tar.gz
    
       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.
       
       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:
            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
                 (c-add-style "pgsql"
                                      '("bsd"
                                     (indent-tabs-mode . t)
                                     (c-basic-offset   . 4)
                                     (tab-width . 4)
                                                 (c-offsets-alist .
                                                ((case-label . +))))
                           t) ; t = set this mode on
    
                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:
                     */
    
       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, 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. 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 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 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(). You can use it for heap_update() 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 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.
       
      11) 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
       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 configure.in, and then run autoconf to
       generate configure.
       
       When configure is run by the user, it tests various OS capabilities,
       stores those in config.status and config.cache, and modifies a list of
       *.in files. For example, if there exists a Makefile.in, configure
       generates a Makefile that contains substitutions for all @var@
       parameters found by configure.
       
       When you need to edit files, make sure you don't waste time modifying
       files generated by configure. Edit the *.in file, and re-run configure
       to recreate the needed file. If you run make distclean from the
       top-level source directory, all files derived by configure are
       removed, so you see only the file contained in the source
       distribution.
       
      12) 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
       entry for your OS. Also, use src/config.guess to add your OS to
       src/template/.similar. You shouldn't match the OS version exactly. The
       configure test will look for an exact OS version number, and if not
       found, find a match without version number. Edit src/configure.in to
       add your new OS. (See configure item above.) You will need to run
       autoconf, or patch src/configure too.
       
       Then, check src/include/port and add your new OS file, with
       appropriate values. Hopefully, there is already locking code in
       src/include/storage/s_lock.h for your CPU. There is also a
       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 piece of the transaction.