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Overview
Comment:Size and performance optimization the readDbPage() routine in the pager.
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SHA3-256: ca9e1875c3a893321d70a131fc4ffc76d169ad05e0b48b7006f53b6b467db4be
User & Date: drh 2017-08-18 21:14:50
Context
2017-08-18
22:30
Another size and performance optimization to readDbPage(). This time we eliminate some unnecessary local variables. check-in: 745bc8de user: drh tags: trunk
21:14
Size and performance optimization the readDbPage() routine in the pager. check-in: ca9e1875 user: drh tags: trunk
16:09
For the unix VFS, avoid an unnecessary stat() system call prior to opening any file in the common case where there are no unused file descriptors. check-in: 3075cfa0 user: drh tags: trunk
Changes
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Changes to src/pager.c.

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  */
  setSectorSize(pPager);
  return rc;
}


/*
** Read the content for page pPg out of the database file and into 

** pPg->pData. A shared lock or greater must be held on the database
** file before this function is called.
**
** If page 1 is read, then the value of Pager.dbFileVers[] is set to
** the value read from the database file.
**
** If an IO error occurs, then the IO error is returned to the caller.
** Otherwise, SQLITE_OK is returned.
*/
static int readDbPage(PgHdr *pPg, u32 iFrame){
  Pager *pPager = pPg->pPager; /* Pager object associated with page pPg */
  Pgno pgno = pPg->pgno;       /* Page number to read */
  int rc = SQLITE_OK;          /* Return code */
  int pgsz = pPager->pageSize; /* Number of bytes to read */


  assert( pPager->eState>=PAGER_READER && !MEMDB );
  assert( isOpen(pPager->fd) );

#ifndef SQLITE_OMIT_WAL



  if( iFrame ){
    /* Try to pull the page from the write-ahead log. */
    rc = sqlite3WalReadFrame(pPager->pWal, iFrame, pgsz, pPg->pData);
  }else
#endif
  {
    i64 iOffset = (pgno-1)*(i64)pPager->pageSize;
    rc = sqlite3OsRead(pPager->fd, pPg->pData, pgsz, iOffset);
    if( rc==SQLITE_IOERR_SHORT_READ ){
      rc = SQLITE_OK;
    }
  }

................................................................................

  assert( pagerUseWal(pPager) );
  pPg = sqlite3PagerLookup(pPager, iPg);
  if( pPg ){
    if( sqlite3PcachePageRefcount(pPg)==1 ){
      sqlite3PcacheDrop(pPg);
    }else{
      u32 iFrame = 0;
      rc = sqlite3WalFindFrame(pPager->pWal, pPg->pgno, &iFrame);
      if( rc==SQLITE_OK ){
        rc = readDbPage(pPg, iFrame);
      }
      if( rc==SQLITE_OK ){
        pPager->xReiniter(pPg);
      }
      sqlite3PagerUnrefNotNull(pPg);
    }
  }

................................................................................
        TESTONLY( rc = ) addToSavepointBitvecs(pPager, pgno);
        testcase( rc==SQLITE_NOMEM );
        sqlite3EndBenignMalloc();
      }
      memset(pPg->pData, 0, pPager->pageSize);
      IOTRACE(("ZERO %p %d\n", pPager, pgno));
    }else{
      u32 iFrame = 0;                 /* Frame to read from WAL file */
      if( pagerUseWal(pPager) ){
        rc = sqlite3WalFindFrame(pPager->pWal, pgno, &iFrame);
        if( rc!=SQLITE_OK ) goto pager_acquire_err;
      }
      assert( pPg->pPager==pPager );
      pPager->aStat[PAGER_STAT_MISS]++;
      rc = readDbPage(pPg, iFrame);
      if( rc!=SQLITE_OK ){
        goto pager_acquire_err;
      }
    }
    pager_set_pagehash(pPg);
  }
  return SQLITE_OK;







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  */
  setSectorSize(pPager);
  return rc;
}


/*
** Read the content for page pPg out of the database file (or out of
** the WAL if that is where the most recent copy if found) into 
** pPg->pData. A shared lock or greater must be held on the database
** file before this function is called.
**
** If page 1 is read, then the value of Pager.dbFileVers[] is set to
** the value read from the database file.
**
** If an IO error occurs, then the IO error is returned to the caller.
** Otherwise, SQLITE_OK is returned.
*/
static int readDbPage(PgHdr *pPg){
  Pager *pPager = pPg->pPager; /* Pager object associated with page pPg */
  Pgno pgno = pPg->pgno;       /* Page number to read */
  int rc = SQLITE_OK;          /* Return code */
  int pgsz = pPager->pageSize; /* Number of bytes to read */
  u32 iFrame = 0;              /* Frame of WAL containing pgno */

  assert( pPager->eState>=PAGER_READER && !MEMDB );
  assert( isOpen(pPager->fd) );

  if( pagerUseWal(pPager) ){
    rc = sqlite3WalFindFrame(pPager->pWal, pgno, &iFrame);
    if( rc ) return rc;
  }
  if( iFrame ){

    rc = sqlite3WalReadFrame(pPager->pWal, iFrame, pgsz, pPg->pData);
  }else{


    i64 iOffset = (pgno-1)*(i64)pPager->pageSize;
    rc = sqlite3OsRead(pPager->fd, pPg->pData, pgsz, iOffset);
    if( rc==SQLITE_IOERR_SHORT_READ ){
      rc = SQLITE_OK;
    }
  }

................................................................................

  assert( pagerUseWal(pPager) );
  pPg = sqlite3PagerLookup(pPager, iPg);
  if( pPg ){
    if( sqlite3PcachePageRefcount(pPg)==1 ){
      sqlite3PcacheDrop(pPg);
    }else{



      rc = readDbPage(pPg);

      if( rc==SQLITE_OK ){
        pPager->xReiniter(pPg);
      }
      sqlite3PagerUnrefNotNull(pPg);
    }
  }

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        TESTONLY( rc = ) addToSavepointBitvecs(pPager, pgno);
        testcase( rc==SQLITE_NOMEM );
        sqlite3EndBenignMalloc();
      }
      memset(pPg->pData, 0, pPager->pageSize);
      IOTRACE(("ZERO %p %d\n", pPager, pgno));
    }else{





      assert( pPg->pPager==pPager );
      pPager->aStat[PAGER_STAT_MISS]++;
      rc = readDbPage(pPg);
      if( rc!=SQLITE_OK ){
        goto pager_acquire_err;
      }
    }
    pager_set_pagehash(pPg);
  }
  return SQLITE_OK;