SQLite

  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 */
#ifdef SQLITE_DIRECT_OVERFLOW_READ
  unsigned char * const pBufStart = pBuf;
  int bEnd;                                 /* True if reading to end of data */
#endif

  assert( pPage );
  assert( pCur->eState==CURSOR_VALID );
  assert( pCur->aiIdx[pCur->iPage]<pPage->nCell );
  assert( cursorHoldsMutex(pCur) );
  assert( eOp!=2 || offset==0 );      /* Always start from beginning for eOp==2 */

        **
        **   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) */
         && pBt->pPage1->aData[19]==0x01                       /* (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));
          nextPage = get4byte(aWrite);
          memcpy(aWrite, aSave, 4);
        }else
#endif

# 2014 October 01
#
# The author disclaims copyright to this source code.  In place of
# a legal notice, here is a blessing:
#
#    May you do good and not evil.
#    May you find forgiveness for yourself and forgive others.
#    May you share freely, never taking more than you give.
#
#***********************************************************************
# This file implements regression tests for SQLite library.  The
# focus of this file is testing the SQLITE_DIRECT_OVERFLOW_READ logic.
#

set testdir [file dirname $argv0]
source $testdir/tester.tcl
set testprefix ovfl

# Populate table t2:
#
#   CREATE TABLE t1(c1 TEXT, c2 TEXT);
#
# with 2000 rows. In each row, c2 spans multiple overflow pages. The text
# value of c1 ranges in size from 1 to 2000 bytes. The idea is to create
# at least one row where the first byte of c2 is also the first byte of
# an overflow page. This was at one point exposing an obscure bug in the
# SQLITE_DIRECT_OVERFLOW_READ logic.
#
do_test 1.1 {
  set c2 [string repeat abcdefghij 200]
  execsql {
    PRAGMA cache_size = 10;
    CREATE TABLE t1(c1 TEXT, c2 TEXT);
    BEGIN;
  }
  for {set i 1} {$i <= 2000} {incr i} {
    set c1 [string repeat . $i]
    execsql { INSERT INTO t1 VALUES($c1, $c2) }
  }
  execsql COMMIT
} {}

do_execsql_test 1.2 {
  SELECT sum(length(c2)) FROM t1;
} [expr 2000 * 2000]

finish_test