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SQLite training in Houston TX on 2019-11-05 (details)
Part of the 2019 Tcl Conference

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Overview
Comment:Make SQLITE_DIRECT_OVERFLOW_READ work for in WAL mode as long as the page being read is not in the the -wal file.
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SHA1: 9879be1899adf5634f551a2077b15ccb1133e4e3
User & Date: drh 2017-01-26 02:26:02
Context
2017-01-26
16:27
Minor simplification and performance optimization for Direct Overflow Read. check-in: 3e96d6ef user: drh tags: trunk
02:26
Make SQLITE_DIRECT_OVERFLOW_READ work for in WAL mode as long as the page being read is not in the the -wal file. check-in: 9879be18 user: drh tags: trunk
01:54
Enhancements to the kvtest utility program. Add the --jmode option. Improved output formatting. check-in: 62a4851c user: drh tags: trunk
Changes
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Changes to src/btree.c.

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#ifdef SQLITE_DIRECT_OVERFLOW_READ
        /* If all the following are true:
        **
        **   1) this is a read operation, and 
        **   2) data is required from the start of this overflow page, and
        **   3) the database is file-backed, and
        **   4) there is no open write-transaction, and
        **   5) the database is not a WAL database,
        **   6) all data from the page is being read.
        **   7) at least 4 bytes have already been read into the output buffer 
        **
        ** then data can be read directly from the database file into the
        ** output buffer, bypassing the page-cache altogether. This speeds
        ** up loading large records that span many overflow pages.
        */
        if( (eOp&0x01)==0                                      /* (1) */
         && offset==0                                          /* (2) */
         && (bEnd || a==ovflSize)                              /* (6) */
         && pBt->inTransaction==TRANS_READ                     /* (4) */
         && (fd = sqlite3PagerFile(pBt->pPager))->pMethods     /* (3) */
         && 0==sqlite3PagerUseWal(pBt->pPager)                 /* (5) */
         && &pBuf[-4]>=pBufStart                               /* (7) */
        ){
          u8 aSave[4];
          u8 *aWrite = &pBuf[-4];
          assert( aWrite>=pBufStart );                         /* hence (7) */
          memcpy(aSave, aWrite, 4);
          rc = sqlite3OsRead(fd, aWrite, a+4, (i64)pBt->pageSize*(nextPage-1));







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#ifdef SQLITE_DIRECT_OVERFLOW_READ
        /* If all the following are true:
        **
        **   1) this is a read operation, and 
        **   2) data is required from the start of this overflow page, and
        **   3) the database is file-backed, and
        **   4) there is no open write-transaction, and
        **   5) the page is not in the WAL file
        **   6) all data from the page is being read.
        **   7) at least 4 bytes have already been read into the output buffer 
        **
        ** then data can be read directly from the database file into the
        ** output buffer, bypassing the page-cache altogether. This speeds
        ** up loading large records that span many overflow pages.
        */
        if( (eOp&0x01)==0                                      /* (1) */
         && offset==0                                          /* (2) */
         && (bEnd || a==ovflSize)                              /* (6) */
         && pBt->inTransaction==TRANS_READ                     /* (4) */
         && (fd = sqlite3PagerFile(pBt->pPager))->pMethods     /* (3) */
         && 0==sqlite3PagerUseWal(pBt->pPager, nextPage)       /* (5) */
         && &pBuf[-4]>=pBufStart                               /* (7) */
        ){
          u8 aSave[4];
          u8 *aWrite = &pBuf[-4];
          assert( aWrite>=pBufStart );                         /* hence (7) */
          memcpy(aSave, aWrite, 4);
          rc = sqlite3OsRead(fd, aWrite, a+4, (i64)pBt->pageSize*(nextPage-1));

Changes to src/pager.c.

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** instead of
**
**   if( pPager->jfd->pMethods ){ ...
*/
#define isOpen(pFd) ((pFd)->pMethods!=0)

/*
** Return true if this pager uses a write-ahead log instead of the usual
** rollback journal. Otherwise false.
*/
#ifndef SQLITE_OMIT_WAL
int sqlite3PagerUseWal(Pager *pPager){


  return (pPager->pWal!=0);


}


# define pagerUseWal(x) sqlite3PagerUseWal(x)
#else
# define pagerUseWal(x) 0
# define pagerRollbackWal(x) 0
# define pagerWalFrames(v,w,x,y) 0
# define pagerOpenWalIfPresent(z) SQLITE_OK
# define pagerBeginReadTransaction(z) SQLITE_OK
#endif







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** instead of
**
**   if( pPager->jfd->pMethods ){ ...
*/
#define isOpen(pFd) ((pFd)->pMethods!=0)

/*
** Return true if this pager uses a write-ahead log to read page pgno.
** Return false if the pager reads pgno directly from the database.
*/
#if !defined(SQLITE_OMIT_WAL) && defined(SQLITE_DIRECT_OVERFLOW_READ)
int sqlite3PagerUseWal(Pager *pPager, Pgno pgno){
  u32 iRead = 0;
  int rc;
  if( pPager->pWal==0 ) return 0;
  rc = sqlite3WalFindFrame(pPager->pWal, pgno, &iRead);
  return rc || iRead;
}
#endif
#ifndef SQLITE_OMIT_WAL
# define pagerUseWal(x) ((x)->pWal!=0)
#else
# define pagerUseWal(x) 0
# define pagerRollbackWal(x) 0
# define pagerWalFrames(v,w,x,y) 0
# define pagerOpenWalIfPresent(z) SQLITE_OK
# define pagerBeginReadTransaction(z) SQLITE_OK
#endif

Changes to src/pager.h.

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#ifndef SQLITE_OMIT_WAL
  int sqlite3PagerCheckpoint(Pager *pPager, sqlite3*, int, int*, int*);
  int sqlite3PagerWalSupported(Pager *pPager);
  int sqlite3PagerWalCallback(Pager *pPager);
  int sqlite3PagerOpenWal(Pager *pPager, int *pisOpen);
  int sqlite3PagerCloseWal(Pager *pPager, sqlite3*);

  int sqlite3PagerUseWal(Pager *pPager);

# ifdef SQLITE_ENABLE_SNAPSHOT
  int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot);
  int sqlite3PagerSnapshotOpen(Pager *pPager, sqlite3_snapshot *pSnapshot);
  int sqlite3PagerSnapshotRecover(Pager *pPager);
# endif
#else
# define sqlite3PagerUseWal(x) 0
#endif

#ifdef SQLITE_ENABLE_ZIPVFS
  int sqlite3PagerWalFramesize(Pager *pPager);
#endif

/* Functions used to query pager state and configuration. */







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#ifndef SQLITE_OMIT_WAL
  int sqlite3PagerCheckpoint(Pager *pPager, sqlite3*, int, int*, int*);
  int sqlite3PagerWalSupported(Pager *pPager);
  int sqlite3PagerWalCallback(Pager *pPager);
  int sqlite3PagerOpenWal(Pager *pPager, int *pisOpen);
  int sqlite3PagerCloseWal(Pager *pPager, sqlite3*);
# ifdef SQLITE_DIRECT_OVERFLOW_READ
  int sqlite3PagerUseWal(Pager *pPager, Pgno);
# endif
# ifdef SQLITE_ENABLE_SNAPSHOT
  int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot);
  int sqlite3PagerSnapshotOpen(Pager *pPager, sqlite3_snapshot *pSnapshot);
  int sqlite3PagerSnapshotRecover(Pager *pPager);
# endif
#else
# define sqlite3PagerUseWal(x,y) 0
#endif

#ifdef SQLITE_ENABLE_ZIPVFS
  int sqlite3PagerWalFramesize(Pager *pPager);
#endif

/* Functions used to query pager state and configuration. */