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SQLite training in Houston TX on 2019-11-05 (details)
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Overview
Comment:Only use the direct-overflow-read optimization if all data from the overflow page in question is being read.
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SHA1: d8e1f75ddf10f3c0b21acd5455f90fdcea54a948
User & Date: dan 2014-03-20 18:56:35
Context
2014-03-31
22:03
Merge in the latest changes and fixes from trunk. check-in: f6211540 user: drh tags: overflow-pgno-cache
2014-03-20
18:56
Only use the direct-overflow-read optimization if all data from the overflow page in question is being read. check-in: d8e1f75d user: dan tags: overflow-pgno-cache
2014-03-11
23:44
Remove a stray C99-style comment. check-in: f500e87d user: drh tags: overflow-pgno-cache
Changes
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Changes to src/btree.c.

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){
  unsigned char *aPayload;
  int rc = SQLITE_OK;
  u32 nKey;
  int iIdx = 0;
  MemPage *pPage = pCur->apPage[pCur->iPage]; /* Btree page of current entry */
  BtShared *pBt = pCur->pBt;                  /* Btree this cursor belongs to */


  assert( pPage );
  assert( pCur->eState==CURSOR_VALID );
  assert( pCur->aiIdx[pCur->iPage]<pPage->nCell );
  assert( cursorHoldsMutex(pCur) );

  getCellInfo(pCur);
  aPayload = pCur->info.pCell + pCur->info.nHeader;
  nKey = (pPage->intKey ? 0 : (int)pCur->info.nKey);


  if( NEVER(offset+amt > nKey+pCur->info.nData) 
   || &aPayload[pCur->info.nLocal] > &pPage->aData[pBt->usableSize]
  ){
    /* Trying to read or write past the end of the data is an error */
    return SQLITE_CORRUPT_BKPT;
  }
................................................................................
        /* 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,

        **
        ** 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) */

         && pBt->inTransaction==TRANS_READ                     /* (4) */
         && (fd = sqlite3PagerFile(pBt->pPager))->pMethods     /* (3) */
         && pBt->pPage1->aData[19]==0x01                       /* (5) */
        ){
          u8 aSave[4];
          u8 *aWrite = &pBuf[-4];
          memcpy(aSave, aWrite, 4);







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){
  unsigned char *aPayload;
  int rc = SQLITE_OK;
  u32 nKey;
  int iIdx = 0;
  MemPage *pPage = pCur->apPage[pCur->iPage]; /* Btree page of current entry */
  BtShared *pBt = pCur->pBt;                  /* Btree this cursor belongs to */
  int bEnd;                       /* True if reading to end of data */

  assert( pPage );
  assert( pCur->eState==CURSOR_VALID );
  assert( pCur->aiIdx[pCur->iPage]<pPage->nCell );
  assert( cursorHoldsMutex(pCur) );

  getCellInfo(pCur);
  aPayload = pCur->info.pCell + pCur->info.nHeader;
  nKey = (pPage->intKey ? 0 : (int)pCur->info.nKey);
  bEnd = (offset+amt==nKey+pCur->info.nData);

  if( NEVER(offset+amt > nKey+pCur->info.nData) 
   || &aPayload[pCur->info.nLocal] > &pPage->aData[pBt->usableSize]
  ){
    /* Trying to read or write past the end of the data is an error */
    return SQLITE_CORRUPT_BKPT;
  }
................................................................................
        /* 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.
        **
        ** 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) */
         && pBt->pPage1->aData[19]==0x01                       /* (5) */
        ){
          u8 aSave[4];
          u8 *aWrite = &pBuf[-4];
          memcpy(aSave, aWrite, 4);

Changes to test/func.test.

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do_test func-29.3 {
  db close
  sqlite3 db test.db
  sqlite3_db_status db CACHE_MISS 1
  db eval {SELECT typeof(+x) FROM t29 ORDER BY id}
} {integer null real blob text}
if {[permutation] != "mmap"} {

  do_test func-29.4 {
    set x [lindex [sqlite3_db_status db CACHE_MISS 1] 1]
    if {$x>100} {set x many}
    set x
  } {many}
}

do_test func-29.5 {
  db close
  sqlite3 db test.db
  sqlite3_db_status db CACHE_MISS 1
  db eval {SELECT sum(length(x)) FROM t29}
} {1000009}
do_test func-29.6 {







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do_test func-29.3 {
  db close
  sqlite3 db test.db
  sqlite3_db_status db CACHE_MISS 1
  db eval {SELECT typeof(+x) FROM t29 ORDER BY id}
} {integer null real blob text}
if {[permutation] != "mmap"} {
  ifcapable !direct_read {
    do_test func-29.4 {
      set x [lindex [sqlite3_db_status db CACHE_MISS 1] 1]
      if {$x>100} {set x many}
      set x
    } {many}
  }
}
do_test func-29.5 {
  db close
  sqlite3 db test.db
  sqlite3_db_status db CACHE_MISS 1
  db eval {SELECT sum(length(x)) FROM t29}
} {1000009}
do_test func-29.6 {