PostgreSQL源码简单分析

PostgreSQL源码简单分析(by [email protected])
   
    PostgreSQL是一个非常强大的开源数据库，既然使开源，当然，我们可以去修改他的代码做任何事情。
最近，忙着设计一个分布式数据库系统，所以，理所当然，就想到了在postgresql的基础上直接改。因此，
分析其源代码就必不可少了。
    简单讲一下分析内容。
    源码目录：
    $ cd postgresql-8.2.4/src/backend/
    $ ls
    access     catalog   executor  libpq  Makefile  nodes      parser  port      postmaster  rewrite  tcop
    bootstrap  commands  lib       main   nls.mk    optimizer  po      postgres  regex       storage  utils
   
    其中：main/main.c是程序启动主文件
          主文件没有作什么重要的事情，主要是作成为daemon等等一些我们并不关心的事情。
         
          tcop/postgres.c是backend执行入口文件。
          请看第3414行：
               case  /* simple query */
    {
     const char *query_string;

     /* Set statement_timestamp() */
     SetCurrentStatementStartTimestamp();

     query_string = pq_getmsgstring(&input_message); //拿到通过libpq传过来的sql语句
     
     pq_getmsgend(&input_message);

     exec_simple_query(query_string); //执行这个sql，并把结果通过libpq返回

     send_ready_for_query = true;
    }
    break;
   
     再看看postgres.c的第745行：
static void
exec_simple_query(const char *query_string)
{
CommandDest dest = whereToSendOutput;
MemoryContext oldcontext;
List    *parsetree_list;
ListCell   *parsetree_item;
bool  save_log_statement_stats = log_statement_stats;
bool  was_logged = false;
char  msec_str[32];

/*
  * Report query to various monitoring facilities.
  */
debug_query_string = query_string;

pgstat_report_activity(query_string);

/*
  * We use save_log_statement_stats so ShowUsage doesn’t report incorrect
  * results because ResetUsage wasn’t called.
  */
if (save_log_statement_stats)
  ResetUsage();

/*
  * Start up a transaction command. All queries generated by the
  * query_string will be in this same command block, *unless* we find a
  * BEGIN/COMMIT/ABORT statement; we have to force a new xact command after
  * one of those, else bad things will happen in xact.c. (Note that this
  * will normally change current memory context.)
  */
start_xact_command();

/*
  * Zap any pre-existing unnamed statement. (While not strictly necessary,
  * it seems best to define simple-Query mode as if it used the unnamed
  * statement and portal; this ensures we recover any storage used by prior
  * unnamed operations.)
  */
unnamed_stmt_pstmt = NULL;
if (unnamed_stmt_context)
{
  DropDependentPortals(unnamed_stmt_context);
  MemoryContextDelete(unnamed_stmt_context);
}
unnamed_stmt_context = NULL;

/*
  * Switch to appropriate context for constructing parsetrees.
  */
oldcontext = MemoryContextSwitchTo(MessageContext);

QueryContext = CurrentMemoryContext;

/*
  * Do basic parsing of the query or queries (this should be safe even if
  * we are in aborted transaction state!)
  */
   
    // 解析这个sql语句到一个语法树结构中
parsetree_list = pg_parse_query(query_string);
   
   
    我想做的事情如下：
    在postgresql的基础上作一个分布式数据库，但sql parse和backend/frontend的通信都不想自己写，
    也就是说要使用postgresql的libpq。
    因此做如下实验：
    任何sql语句进来后，我会在exec_simple_query里面捷获，如果是一个select语句，
    我会返回一行记录：列名—name   列值– lijianghua
   
继续分析文件： src/access/common/printtup.c

//以下函数使通过libpq发送返回的列的column 描述信息的
void
SendRowDescriptionMessage(TupleDesc typeinfo, List *targetlist, int16 *formats)
{
Form_pg_attribute *attrs = typeinfo->attrs;
int   natts = typeinfo->natts;
int   proto = PG_PROTOCOL_MAJOR(FrontendProtocol);
int   i;
StringInfoData buf;
ListCell   *tlist_item = list_head(targetlist);

pq_beginmessage(&buf, ‘T’); /* tuple descriptor message type */
pq_sendint(&buf, natts, 2); /* # of attrs in tuples */

for (i = 0; i < natts; ++i)
{
  Oid   atttypid = attrs->atttypid;
  int32  atttypmod = attrs->atttypmod;

  pq_sendstring(&buf, NameStr(attrs->attname));
  /* column ID info appears in protocol 3.0 and up */
  if (proto >= 3)
  {
   /* Do we have a non-resjunk tlist item? */
   while (tlist_item &&
       ((TargetEntry *) lfirst(tlist_item))->resjunk)
    tlist_item = lnext(tlist_item);
   if (tlist_item)
   {
    TargetEntry *tle = (TargetEntry *) lfirst(tlist_item);

    pq_sendint(&buf, tle->resorigtbl, 4);
    pq_sendint(&buf, tle->resorigcol, 2);
    tlist_item = lnext(tlist_item);
   }
   else
   {
    /* No info available, so send zeroes */
    pq_sendint(&buf, 0, 4);
    pq_sendint(&buf, 0, 2);
   }
  }
  /* If column is a domain, send the base type and typmod instead */
  atttypid = getBaseTypeAndTypmod(atttypid, &atttypmod);
  pq_sendint(&buf, (int) atttypid, sizeof(atttypid));
  pq_sendint(&buf, attrs->attlen, sizeof(attrs->attlen));
  /* typmod appears in protocol 2.0 and up */
  if (proto >= 2)
   pq_sendint(&buf, atttypmod, sizeof(atttypmod));
  /* format info appears in protocol 3.0 and up */
  if (proto >= 3)
  {
   if (formats)
    pq_sendint(&buf, formats, 2);
   else
    pq_sendint(&buf, 0, 2);
  }
}
pq_endmessage(&buf);
}

//下面这个函数是select返回的数据的值，每一行数据都会调用一下这个函数
static void
printtup(TupleTableSlot *slot, DestReceiver *self)
{
TupleDesc typeinfo = slot->tts_tupleDescriptor;
DR_printtup *myState = (DR_printtup *) self;
StringInfoData buf;
int   natts = typeinfo->natts;
int   i;

/* Set or update my derived attribute info, if needed */
if (myState->attrinfo != typeinfo || myState->nattrs != natts)
  printtup_prepare_info(myState, typeinfo, natts);

/* Make sure the tuple is fully deconstructed */
slot_getallattrs(slot);

/*
  * Prepare a DataRow message
  */
pq_beginmessage(&buf, ‘D’);

pq_sendint(&buf, natts, 2);

/*
  * send the attributes of this tuple
  */
for (i = 0; i < natts; ++i)
{
  PrinttupAttrInfo *thisState = myState->myinfo + i;
  Datum  origattr = slot->tts_values,
     attr;

  if (slot->tts_isnull)
  {
   pq_sendint(&buf, -1, 4);
   continue;
  }

  /*
   * If we have a toasted datum, forcibly detoast it here to avoid
   * memory leakage inside the type’s output routine.
   */
  if (thisState->typisvarlena)
   attr = PointerGetDatum(PG_DETOAST_DATUM(origattr));
  else
   attr = origattr;

  if (thisState->format == 0)
  {
   /* Text output */
   char    *outputstr;

   outputstr = OutputFunctionCall(&thisState->finfo, attr);
   pq_sendcountedtext(&buf, outputstr, strlen(outputstr), false);
   pfree(outputstr);
  }
  else
  {
   /* Binary output */
   bytea    *outputbytes;

   outputbytes = SendFunctionCall(&thisState->finfo, attr);
   pq_sendint(&buf, VARSIZE(outputbytes) - VARHDRSZ, 4);
   pq_sendbytes(&buf, VARDATA(outputbytes),
       VARSIZE(outputbytes) - VARHDRSZ);
   pfree(outputbytes);
  }

  /* Clean up detoasted copy, if any */
  if (attr != origattr)
   pfree(DatumGetPointer(attr));
}

pq_endmessage(&buf);
}

根据以上分析，我来修改exec_simple_query:
   在833行加入如下内容：
   //此范例只处理select语句
   if(parsetree->type == T_SelectStmt)
   {
        StringInfoData buf;
        pq_beginmessage(&buf, ‘T’);  /* tuple descriptor message type */
        pq_sendint(&buf, 1, 2);      /* number of columns in tuples */
        pq_sendstring(&buf, “name”); // column名称
        pq_sendint(&buf, 0, 4);     
        pq_sendint(&buf, 0, 2);
        pq_sendint(&buf, 0, 4);
        pq_sendint(&buf, 2, 2);
        pq_sendint(&buf, 0, 4);
        pq_sendint(&buf, 0, 2);
        pq_endmessage(&buf);
      
        pq_beginmessage(&buf, ‘D’);
        pq_sendint(&buf, 1, 2);
        pq_sendcountedtext(&buf, “lijianghua”, 10, false);
        pq_endmessage(&buf);
      
        //此行必须加上,告诉libpq返回结果结束(C代表completed)
        pq_puttextmessage(’C', “select return 1 rows”);
      
        return;
   }
  
   修改结束，按照正常流程编译PostgreSQL,并启动。
   测试结果：
[mypg@webtrends /copyright for distribution terms
       /h for help with SQL commands
       /? for help with psql commands
       /g or terminate with semicolon to execute query
       /q to quit

mypg=# /d
List of relations
    name   
————
lijianghua
(1 row)

mypg=# select * from test2;
    name   
————
lijianghua
(1 row)

mypg=# select * from test3;
    name   
————
lijianghua
(1 row)

mypg=# select * from test5;
    name   
————
lijianghua
(1 row)

    可以看到任何select语句都只返回我们预定义的结果，说明我们当初的想法是可行的(/d其实也是一个select语句)。
    下一目标是基于postgresql自己开发一个分布式数据库，有得忙了。