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Overview
Comment:Change the Pager.hasBeenUsed flag into Pager.hasHeldSharedLock in order to take it off the critical path in sqlite3PagerAcquire().
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SHA1: b79096be7cb02aae2f303db33a8bf19e69204374
User & Date: drh 2015-09-03 17:54:32
Context
2015-09-03
18:20
A simple optimization and size reduction in sqlite3PagerAcquire(). check-in: 618d8dd4 user: drh tags: trunk
17:54
Change the Pager.hasBeenUsed flag into Pager.hasHeldSharedLock in order to take it off the critical path in sqlite3PagerAcquire(). check-in: b79096be user: drh tags: trunk
13:46
Add the sqlite3VdbeLoadString() and sqlite3VdbeMultiLoad() routines to help with code generation, especially on PRAGMAs. Rename sqlite3VdbeAddGoto() to just sqlite3VdbeGoto(). check-in: 847387ec user: drh tags: trunk
Changes
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Changes to src/pager.c.

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  u8 eState;                  /* Pager state (OPEN, READER, WRITER_LOCKED..) */
  u8 eLock;                   /* Current lock held on database file */
  u8 changeCountDone;         /* Set after incrementing the change-counter */
  u8 setMaster;               /* True if a m-j name has been written to jrnl */
  u8 doNotSpill;              /* Do not spill the cache when non-zero */
  u8 subjInMemory;            /* True to use in-memory sub-journals */
  u8 bUseFetch;               /* True to use xFetch() */
  u8 hasBeenUsed;             /* True if any content previously read */
  Pgno dbSize;                /* Number of pages in the database */
  Pgno dbOrigSize;            /* dbSize before the current transaction */
  Pgno dbFileSize;            /* Number of pages in the database file */
  Pgno dbHintSize;            /* Value passed to FCNTL_SIZE_HINT call */
  int errCode;                /* One of several kinds of errors */
  int nRec;                   /* Pages journalled since last j-header written */
  u32 cksumInit;              /* Quasi-random value added to every checksum */
................................................................................

      assert( pPager->eState==PAGER_OPEN );
      assert( (pPager->eLock==SHARED_LOCK)
           || (pPager->exclusiveMode && pPager->eLock>SHARED_LOCK)
      );
    }

    if( !pPager->tempFile && pPager->hasBeenUsed ){
      /* The shared-lock has just been acquired then check to
      ** see if the database has been modified.  If the database has changed,
      ** flush the cache.  The pPager->hasBeenUsed flag prevents this from
      ** occurring on the very first access to a file, in order to save a
      ** single unnecessary sqlite3OsRead() call at the start-up.
      **
      ** Database changes are detected by looking at 15 bytes beginning
      ** at offset 24 into the file.  The first 4 of these 16 bytes are
      ** a 32-bit counter that is incremented with each change.  The
      ** other bytes change randomly with each file change when
................................................................................
 failed:
  if( rc!=SQLITE_OK ){
    assert( !MEMDB );
    pager_unlock(pPager);
    assert( pPager->eState==PAGER_OPEN );
  }else{
    pPager->eState = PAGER_READER;

  }
  return rc;
}

/*
** If the reference count has reached zero, rollback any active
** transaction and unlock the pager.
................................................................................
  assert( pPager->eState>=PAGER_READER );
  assert( assert_pager_state(pPager) );
  assert( noContent==0 || bMmapOk==0 );

  if( pgno==0 ){
    return SQLITE_CORRUPT_BKPT;
  }
  pPager->hasBeenUsed = 1;

  /* If the pager is in the error state, return an error immediately. 
  ** Otherwise, request the page from the PCache layer. */
  if( pPager->errCode!=SQLITE_OK ){
    rc = pPager->errCode;
  }else{
    if( bMmapOk && pagerUseWal(pPager) ){
................................................................................
*/
DbPage *sqlite3PagerLookup(Pager *pPager, Pgno pgno){
  sqlite3_pcache_page *pPage;
  assert( pPager!=0 );
  assert( pgno!=0 );
  assert( pPager->pPCache!=0 );
  pPage = sqlite3PcacheFetch(pPager->pPCache, pgno, 0);
  assert( pPage==0 || pPager->hasBeenUsed );
  if( pPage==0 ) return 0;
  return sqlite3PcacheFetchFinish(pPager->pPCache, pgno, pPage);
}

/*
** Release a page reference.
**







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  u8 eState;                  /* Pager state (OPEN, READER, WRITER_LOCKED..) */
  u8 eLock;                   /* Current lock held on database file */
  u8 changeCountDone;         /* Set after incrementing the change-counter */
  u8 setMaster;               /* True if a m-j name has been written to jrnl */
  u8 doNotSpill;              /* Do not spill the cache when non-zero */
  u8 subjInMemory;            /* True to use in-memory sub-journals */
  u8 bUseFetch;               /* True to use xFetch() */
  u8 hasHeldSharedLock;       /* True if a shared lock has ever been held */
  Pgno dbSize;                /* Number of pages in the database */
  Pgno dbOrigSize;            /* dbSize before the current transaction */
  Pgno dbFileSize;            /* Number of pages in the database file */
  Pgno dbHintSize;            /* Value passed to FCNTL_SIZE_HINT call */
  int errCode;                /* One of several kinds of errors */
  int nRec;                   /* Pages journalled since last j-header written */
  u32 cksumInit;              /* Quasi-random value added to every checksum */
................................................................................

      assert( pPager->eState==PAGER_OPEN );
      assert( (pPager->eLock==SHARED_LOCK)
           || (pPager->exclusiveMode && pPager->eLock>SHARED_LOCK)
      );
    }

    if( !pPager->tempFile && pPager->hasHeldSharedLock ){
      /* The shared-lock has just been acquired then check to
      ** see if the database has been modified.  If the database has changed,
      ** flush the cache.  The hasHeldSharedLock flag prevents this from
      ** occurring on the very first access to a file, in order to save a
      ** single unnecessary sqlite3OsRead() call at the start-up.
      **
      ** Database changes are detected by looking at 15 bytes beginning
      ** at offset 24 into the file.  The first 4 of these 16 bytes are
      ** a 32-bit counter that is incremented with each change.  The
      ** other bytes change randomly with each file change when
................................................................................
 failed:
  if( rc!=SQLITE_OK ){
    assert( !MEMDB );
    pager_unlock(pPager);
    assert( pPager->eState==PAGER_OPEN );
  }else{
    pPager->eState = PAGER_READER;
    pPager->hasHeldSharedLock = 1;
  }
  return rc;
}

/*
** If the reference count has reached zero, rollback any active
** transaction and unlock the pager.
................................................................................
  assert( pPager->eState>=PAGER_READER );
  assert( assert_pager_state(pPager) );
  assert( noContent==0 || bMmapOk==0 );

  if( pgno==0 ){
    return SQLITE_CORRUPT_BKPT;
  }
  assert( pPager->hasHeldSharedLock==1 );

  /* If the pager is in the error state, return an error immediately. 
  ** Otherwise, request the page from the PCache layer. */
  if( pPager->errCode!=SQLITE_OK ){
    rc = pPager->errCode;
  }else{
    if( bMmapOk && pagerUseWal(pPager) ){
................................................................................
*/
DbPage *sqlite3PagerLookup(Pager *pPager, Pgno pgno){
  sqlite3_pcache_page *pPage;
  assert( pPager!=0 );
  assert( pgno!=0 );
  assert( pPager->pPCache!=0 );
  pPage = sqlite3PcacheFetch(pPager->pPCache, pgno, 0);
  assert( pPage==0 || pPager->hasHeldSharedLock );
  if( pPage==0 ) return 0;
  return sqlite3PcacheFetchFinish(pPager->pPCache, pgno, pPage);
}

/*
** Release a page reference.
**