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Overview
Comment:Performance optimization to the lookaside-memory disabling mechanism.
Downloads: Tarball | ZIP archive | SQL archive
Timelines: family | ancestors | descendants | both | trunk
Files: files | file ages | folders
SHA3-256: 17ce1c49cb0ed6fdd8c7c33431bc2afa217f1092c99b8dd608890a8e7aec7fb1
User & Date: drh 2019-10-05 14:39:36
Context
2019-10-05
15:28
Reorder two comparisons for a small performance gain in OP_Transaction. check-in: d7667f65 user: drh tags: trunk
14:39
Performance optimization to the lookaside-memory disabling mechanism. check-in: 17ce1c49 user: drh tags: trunk
2019-10-04
15:03
Version 3.30.0 check-in: c20a3533 user: drh tags: trunk, release, version-3.30.0
Changes
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Changes to src/analyze.c.

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    Index *pIdx = sqliteHashData(i);
    if( !pIdx->hasStat1 ) sqlite3DefaultRowEst(pIdx);
  }

  /* Load the statistics from the sqlite_stat4 table. */
#ifdef SQLITE_ENABLE_STAT4
  if( rc==SQLITE_OK ){
    db->lookaside.bDisable++;
    rc = loadStat4(db, sInfo.zDatabase);
    db->lookaside.bDisable--;
  }
  for(i=sqliteHashFirst(&pSchema->idxHash); i; i=sqliteHashNext(i)){
    Index *pIdx = sqliteHashData(i);
    sqlite3_free(pIdx->aiRowEst);
    pIdx->aiRowEst = 0;
  }
#endif







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    Index *pIdx = sqliteHashData(i);
    if( !pIdx->hasStat1 ) sqlite3DefaultRowEst(pIdx);
  }

  /* Load the statistics from the sqlite_stat4 table. */
#ifdef SQLITE_ENABLE_STAT4
  if( rc==SQLITE_OK ){
    DisableLookaside;
    rc = loadStat4(db, sInfo.zDatabase);
    EnableLookaside;
  }
  for(i=sqliteHashFirst(&pSchema->idxHash); i; i=sqliteHashNext(i)){
    Index *pIdx = sqliteHashData(i);
    sqlite3_free(pIdx->aiRowEst);
    pIdx->aiRowEst = 0;
  }
#endif

Changes to src/build.c.

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#ifndef SQLITE_OMIT_ALTERTABLE
    u8 eParseMode = pParse->eParseMode;
    pParse->eParseMode = PARSE_MODE_NORMAL;
#endif
    n = pParse->nTab;
    sqlite3SrcListAssignCursors(pParse, pSel->pSrc);
    pTable->nCol = -1;
    db->lookaside.bDisable++;
#ifndef SQLITE_OMIT_AUTHORIZATION
    xAuth = db->xAuth;
    db->xAuth = 0;
    pSelTab = sqlite3ResultSetOfSelect(pParse, pSel, SQLITE_AFF_NONE);
    db->xAuth = xAuth;
#else
    pSelTab = sqlite3ResultSetOfSelect(pParse, pSel, SQLITE_AFF_NONE);
................................................................................
      assert( sqlite3SchemaMutexHeld(db, 0, pTable->pSchema) );
    }else{
      pTable->nCol = 0;
      nErr++;
    }
    sqlite3DeleteTable(db, pSelTab);
    sqlite3SelectDelete(db, pSel);
    db->lookaside.bDisable--;
#ifndef SQLITE_OMIT_ALTERTABLE
    pParse->eParseMode = eParseMode;
#endif
  } else {
    nErr++;
  }
  pTable->pSchema->schemaFlags |= DB_UnresetViews;







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#ifndef SQLITE_OMIT_ALTERTABLE
    u8 eParseMode = pParse->eParseMode;
    pParse->eParseMode = PARSE_MODE_NORMAL;
#endif
    n = pParse->nTab;
    sqlite3SrcListAssignCursors(pParse, pSel->pSrc);
    pTable->nCol = -1;
    DisableLookaside;
#ifndef SQLITE_OMIT_AUTHORIZATION
    xAuth = db->xAuth;
    db->xAuth = 0;
    pSelTab = sqlite3ResultSetOfSelect(pParse, pSel, SQLITE_AFF_NONE);
    db->xAuth = xAuth;
#else
    pSelTab = sqlite3ResultSetOfSelect(pParse, pSel, SQLITE_AFF_NONE);
................................................................................
      assert( sqlite3SchemaMutexHeld(db, 0, pTable->pSchema) );
    }else{
      pTable->nCol = 0;
      nErr++;
    }
    sqlite3DeleteTable(db, pSelTab);
    sqlite3SelectDelete(db, pSel);
    EnableLookaside;
#ifndef SQLITE_OMIT_ALTERTABLE
    pParse->eParseMode = eParseMode;
#endif
  } else {
    nErr++;
  }
  pTable->pSchema->schemaFlags |= DB_UnresetViews;

Changes to src/fkey.c.

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          pWhere,
          0, 0, 0, 0, 0
      );
      pWhere = 0;
    }

    /* Disable lookaside memory allocation */
    db->lookaside.bDisable++;

    pTrigger = (Trigger *)sqlite3DbMallocZero(db, 
        sizeof(Trigger) +         /* struct Trigger */
        sizeof(TriggerStep) +     /* Single step in trigger program */
        nFrom + 1                 /* Space for pStep->zTarget */
    );
    if( pTrigger ){
................................................................................
      if( pWhen ){
        pWhen = sqlite3PExpr(pParse, TK_NOT, pWhen, 0);
        pTrigger->pWhen = sqlite3ExprDup(db, pWhen, EXPRDUP_REDUCE);
      }
    }

    /* Re-enable the lookaside buffer, if it was disabled earlier. */
    db->lookaside.bDisable--;

    sqlite3ExprDelete(db, pWhere);
    sqlite3ExprDelete(db, pWhen);
    sqlite3ExprListDelete(db, pList);
    sqlite3SelectDelete(db, pSelect);
    if( db->mallocFailed==1 ){
      fkTriggerDelete(db, pTrigger);







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          pWhere,
          0, 0, 0, 0, 0
      );
      pWhere = 0;
    }

    /* Disable lookaside memory allocation */
    DisableLookaside;

    pTrigger = (Trigger *)sqlite3DbMallocZero(db, 
        sizeof(Trigger) +         /* struct Trigger */
        sizeof(TriggerStep) +     /* Single step in trigger program */
        nFrom + 1                 /* Space for pStep->zTarget */
    );
    if( pTrigger ){
................................................................................
      if( pWhen ){
        pWhen = sqlite3PExpr(pParse, TK_NOT, pWhen, 0);
        pTrigger->pWhen = sqlite3ExprDup(db, pWhen, EXPRDUP_REDUCE);
      }
    }

    /* Re-enable the lookaside buffer, if it was disabled earlier. */
    EnableLookaside;

    sqlite3ExprDelete(db, pWhere);
    sqlite3ExprDelete(db, pWhen);
    sqlite3ExprListDelete(db, pList);
    sqlite3SelectDelete(db, pSelect);
    if( db->mallocFailed==1 ){
      fkTriggerDelete(db, pTrigger);

Changes to src/main.c.

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  }else{
    pStart = pBuf;
  }
  db->lookaside.pStart = pStart;
  db->lookaside.pInit = 0;
  db->lookaside.pFree = 0;
  db->lookaside.sz = (u16)sz;

  if( pStart ){
    int i;
    LookasideSlot *p;
    assert( sz > (int)sizeof(LookasideSlot*) );
    db->lookaside.nSlot = cnt;
    p = (LookasideSlot*)pStart;
    for(i=cnt-1; i>=0; i--){
................................................................................
    db->lookaside.pEnd = p;
    db->lookaside.bDisable = 0;
    db->lookaside.bMalloced = pBuf==0 ?1:0;
  }else{
    db->lookaside.pStart = db;
    db->lookaside.pEnd = db;
    db->lookaside.bDisable = 1;

    db->lookaside.bMalloced = 0;
    db->lookaside.nSlot = 0;
  }
#endif /* SQLITE_OMIT_LOOKASIDE */
  return SQLITE_OK;
}

................................................................................
  }
  sqlite3_mutex_enter(db->mutex);
  db->errMask = 0xff;
  db->nDb = 2;
  db->magic = SQLITE_MAGIC_BUSY;
  db->aDb = db->aDbStatic;
  db->lookaside.bDisable = 1;


  assert( sizeof(db->aLimit)==sizeof(aHardLimit) );
  memcpy(db->aLimit, aHardLimit, sizeof(db->aLimit));
  db->aLimit[SQLITE_LIMIT_WORKER_THREADS] = SQLITE_DEFAULT_WORKER_THREADS;
  db->autoCommit = 1;
  db->nextAutovac = -1;
  db->szMmap = sqlite3GlobalConfig.szMmap;







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  }else{
    pStart = pBuf;
  }
  db->lookaside.pStart = pStart;
  db->lookaside.pInit = 0;
  db->lookaside.pFree = 0;
  db->lookaside.sz = (u16)sz;
  db->lookaside.szTrue = (u16)sz;
  if( pStart ){
    int i;
    LookasideSlot *p;
    assert( sz > (int)sizeof(LookasideSlot*) );
    db->lookaside.nSlot = cnt;
    p = (LookasideSlot*)pStart;
    for(i=cnt-1; i>=0; i--){
................................................................................
    db->lookaside.pEnd = p;
    db->lookaside.bDisable = 0;
    db->lookaside.bMalloced = pBuf==0 ?1:0;
  }else{
    db->lookaside.pStart = db;
    db->lookaside.pEnd = db;
    db->lookaside.bDisable = 1;
    db->lookaside.sz = 0;
    db->lookaside.bMalloced = 0;
    db->lookaside.nSlot = 0;
  }
#endif /* SQLITE_OMIT_LOOKASIDE */
  return SQLITE_OK;
}

................................................................................
  }
  sqlite3_mutex_enter(db->mutex);
  db->errMask = 0xff;
  db->nDb = 2;
  db->magic = SQLITE_MAGIC_BUSY;
  db->aDb = db->aDbStatic;
  db->lookaside.bDisable = 1;
  db->lookaside.sz = 0;

  assert( sizeof(db->aLimit)==sizeof(aHardLimit) );
  memcpy(db->aLimit, aHardLimit, sizeof(db->aLimit));
  db->aLimit[SQLITE_LIMIT_WORKER_THREADS] = SQLITE_DEFAULT_WORKER_THREADS;
  db->autoCommit = 1;
  db->nextAutovac = -1;
  db->szMmap = sqlite3GlobalConfig.szMmap;

Changes to src/malloc.c.

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      assert( sqlite3MemdebugHasType(p, (MEMTYPE_LOOKASIDE|MEMTYPE_HEAP)) );
      assert( sqlite3MemdebugNoType(p, (u8)~(MEMTYPE_LOOKASIDE|MEMTYPE_HEAP)) );
    }
#endif
    return sqlite3GlobalConfig.m.xSize(p);
  }else{
    assert( sqlite3_mutex_held(db->mutex) );
    return db->lookaside.sz;
  }
}
sqlite3_uint64 sqlite3_msize(void *p){
  assert( sqlite3MemdebugNoType(p, (u8)~MEMTYPE_HEAP) );
  assert( sqlite3MemdebugHasType(p, MEMTYPE_HEAP) );
  return p ? sqlite3GlobalConfig.m.xSize(p) : 0;
}
................................................................................
      measureAllocationSize(db, p);
      return;
    }
    if( isLookaside(db, p) ){
      LookasideSlot *pBuf = (LookasideSlot*)p;
#ifdef SQLITE_DEBUG
      /* Trash all content in the buffer being freed */
      memset(p, 0xaa, db->lookaside.sz);
#endif
      pBuf->pNext = db->lookaside.pFree;
      db->lookaside.pFree = pBuf;
      return;
    }
  }
  assert( sqlite3MemdebugHasType(p, (MEMTYPE_LOOKASIDE|MEMTYPE_HEAP)) );
................................................................................
}
void *sqlite3DbMallocRawNN(sqlite3 *db, u64 n){
#ifndef SQLITE_OMIT_LOOKASIDE
  LookasideSlot *pBuf;
  assert( db!=0 );
  assert( sqlite3_mutex_held(db->mutex) );
  assert( db->pnBytesFreed==0 );

  if( db->lookaside.bDisable==0 ){
    assert( db->mallocFailed==0 );
    if( n>db->lookaside.sz ){

      db->lookaside.anStat[1]++;
    }else if( (pBuf = db->lookaside.pFree)!=0 ){
      db->lookaside.pFree = pBuf->pNext;
      db->lookaside.anStat[0]++;
      return (void*)pBuf;
    }else if( (pBuf = db->lookaside.pInit)!=0 ){
      db->lookaside.pInit = pBuf->pNext;
      db->lookaside.anStat[0]++;
      return (void*)pBuf;
    }else{
      db->lookaside.anStat[2]++;
    }
  }else if( db->mallocFailed ){
    return 0;
  }
#else
  assert( db!=0 );
  assert( sqlite3_mutex_held(db->mutex) );
  assert( db->pnBytesFreed==0 );
  if( db->mallocFailed ){
    return 0;
  }
................................................................................
** Resize the block of memory pointed to by p to n bytes. If the
** resize fails, set the mallocFailed flag in the connection object.
*/
void *sqlite3DbRealloc(sqlite3 *db, void *p, u64 n){
  assert( db!=0 );
  if( p==0 ) return sqlite3DbMallocRawNN(db, n);
  assert( sqlite3_mutex_held(db->mutex) );
  if( isLookaside(db,p) && n<=db->lookaside.sz ) return p;
  return dbReallocFinish(db, p, n);
}
static SQLITE_NOINLINE void *dbReallocFinish(sqlite3 *db, void *p, u64 n){
  void *pNew = 0;
  assert( db!=0 );
  assert( p!=0 );
  if( db->mallocFailed==0 ){
    if( isLookaside(db, p) ){
      pNew = sqlite3DbMallocRawNN(db, n);
      if( pNew ){
        memcpy(pNew, p, db->lookaside.sz);
        sqlite3DbFree(db, p);
      }
    }else{
      assert( sqlite3MemdebugHasType(p, (MEMTYPE_LOOKASIDE|MEMTYPE_HEAP)) );
      assert( sqlite3MemdebugNoType(p, (u8)~(MEMTYPE_LOOKASIDE|MEMTYPE_HEAP)) );
      sqlite3MemdebugSetType(p, MEMTYPE_HEAP);
      pNew = sqlite3_realloc64(p, n);
................................................................................
*/
void sqlite3OomFault(sqlite3 *db){
  if( db->mallocFailed==0 && db->bBenignMalloc==0 ){
    db->mallocFailed = 1;
    if( db->nVdbeExec>0 ){
      db->u1.isInterrupted = 1;
    }
    db->lookaside.bDisable++;
    if( db->pParse ){
      db->pParse->rc = SQLITE_NOMEM_BKPT;
    }
  }
}

/*
................................................................................
** VDBEs.
*/
void sqlite3OomClear(sqlite3 *db){
  if( db->mallocFailed && db->nVdbeExec==0 ){
    db->mallocFailed = 0;
    db->u1.isInterrupted = 0;
    assert( db->lookaside.bDisable>0 );
    db->lookaside.bDisable--;
  }
}

/*
** Take actions at the end of an API call to indicate an OOM error
*/
static SQLITE_NOINLINE int apiOomError(sqlite3 *db){







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      assert( sqlite3MemdebugHasType(p, (MEMTYPE_LOOKASIDE|MEMTYPE_HEAP)) );
      assert( sqlite3MemdebugNoType(p, (u8)~(MEMTYPE_LOOKASIDE|MEMTYPE_HEAP)) );
    }
#endif
    return sqlite3GlobalConfig.m.xSize(p);
  }else{
    assert( sqlite3_mutex_held(db->mutex) );
    return db->lookaside.szTrue;
  }
}
sqlite3_uint64 sqlite3_msize(void *p){
  assert( sqlite3MemdebugNoType(p, (u8)~MEMTYPE_HEAP) );
  assert( sqlite3MemdebugHasType(p, MEMTYPE_HEAP) );
  return p ? sqlite3GlobalConfig.m.xSize(p) : 0;
}
................................................................................
      measureAllocationSize(db, p);
      return;
    }
    if( isLookaside(db, p) ){
      LookasideSlot *pBuf = (LookasideSlot*)p;
#ifdef SQLITE_DEBUG
      /* Trash all content in the buffer being freed */
      memset(p, 0xaa, db->lookaside.szTrue);
#endif
      pBuf->pNext = db->lookaside.pFree;
      db->lookaside.pFree = pBuf;
      return;
    }
  }
  assert( sqlite3MemdebugHasType(p, (MEMTYPE_LOOKASIDE|MEMTYPE_HEAP)) );
................................................................................
}
void *sqlite3DbMallocRawNN(sqlite3 *db, u64 n){
#ifndef SQLITE_OMIT_LOOKASIDE
  LookasideSlot *pBuf;
  assert( db!=0 );
  assert( sqlite3_mutex_held(db->mutex) );
  assert( db->pnBytesFreed==0 );
  if( n>db->lookaside.sz ){
    if( db->lookaside.bDisable ){
      return db->mallocFailed ? 0 : dbMallocRawFinish(db, n);

    }
    db->lookaside.anStat[1]++;
  }else if( (pBuf = db->lookaside.pFree)!=0 ){
    db->lookaside.pFree = pBuf->pNext;
    db->lookaside.anStat[0]++;
    return (void*)pBuf;
  }else if( (pBuf = db->lookaside.pInit)!=0 ){
    db->lookaside.pInit = pBuf->pNext;
    db->lookaside.anStat[0]++;
    return (void*)pBuf;
  }else{
    db->lookaside.anStat[2]++;
  }



#else
  assert( db!=0 );
  assert( sqlite3_mutex_held(db->mutex) );
  assert( db->pnBytesFreed==0 );
  if( db->mallocFailed ){
    return 0;
  }
................................................................................
** Resize the block of memory pointed to by p to n bytes. If the
** resize fails, set the mallocFailed flag in the connection object.
*/
void *sqlite3DbRealloc(sqlite3 *db, void *p, u64 n){
  assert( db!=0 );
  if( p==0 ) return sqlite3DbMallocRawNN(db, n);
  assert( sqlite3_mutex_held(db->mutex) );
  if( isLookaside(db,p) && n<=db->lookaside.szTrue ) return p;
  return dbReallocFinish(db, p, n);
}
static SQLITE_NOINLINE void *dbReallocFinish(sqlite3 *db, void *p, u64 n){
  void *pNew = 0;
  assert( db!=0 );
  assert( p!=0 );
  if( db->mallocFailed==0 ){
    if( isLookaside(db, p) ){
      pNew = sqlite3DbMallocRawNN(db, n);
      if( pNew ){
        memcpy(pNew, p, db->lookaside.szTrue);
        sqlite3DbFree(db, p);
      }
    }else{
      assert( sqlite3MemdebugHasType(p, (MEMTYPE_LOOKASIDE|MEMTYPE_HEAP)) );
      assert( sqlite3MemdebugNoType(p, (u8)~(MEMTYPE_LOOKASIDE|MEMTYPE_HEAP)) );
      sqlite3MemdebugSetType(p, MEMTYPE_HEAP);
      pNew = sqlite3_realloc64(p, n);
................................................................................
*/
void sqlite3OomFault(sqlite3 *db){
  if( db->mallocFailed==0 && db->bBenignMalloc==0 ){
    db->mallocFailed = 1;
    if( db->nVdbeExec>0 ){
      db->u1.isInterrupted = 1;
    }
    DisableLookaside;
    if( db->pParse ){
      db->pParse->rc = SQLITE_NOMEM_BKPT;
    }
  }
}

/*
................................................................................
** VDBEs.
*/
void sqlite3OomClear(sqlite3 *db){
  if( db->mallocFailed && db->nVdbeExec==0 ){
    db->mallocFailed = 0;
    db->u1.isInterrupted = 0;
    assert( db->lookaside.bDisable>0 );
    EnableLookaside;
  }
}

/*
** Take actions at the end of an API call to indicate an OOM error
*/
static SQLITE_NOINLINE int apiOomError(sqlite3 *db){

Changes to src/parse.y.

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struct FrameBound     { int eType; Expr *pExpr; };

/*
** Disable lookaside memory allocation for objects that might be
** shared across database connections.
*/
static void disableLookaside(Parse *pParse){

  pParse->disableLookaside++;
  pParse->db->lookaside.bDisable++;

}

} // end %include

// Input is a single SQL command
input ::= cmdlist.
cmdlist ::= cmdlist ecmd.







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struct FrameBound     { int eType; Expr *pExpr; };

/*
** Disable lookaside memory allocation for objects that might be
** shared across database connections.
*/
static void disableLookaside(Parse *pParse){
  sqlite3 *db = pParse->db;
  pParse->disableLookaside++;

  DisableLookaside;
}

} // end %include

// Input is a single SQL command
input ::= cmdlist.
cmdlist ::= cmdlist ecmd.

Changes to src/prepare.c.

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void sqlite3ParserReset(Parse *pParse){
  sqlite3 *db = pParse->db;
  sqlite3DbFree(db, pParse->aLabel);
  sqlite3ExprListDelete(db, pParse->pConstExpr);
  if( db ){
    assert( db->lookaside.bDisable >= pParse->disableLookaside );
    db->lookaside.bDisable -= pParse->disableLookaside;

  }
  pParse->disableLookaside = 0;
}

/*
** Compile the UTF-8 encoded SQL statement zSql into a statement handle.
*/
................................................................................
  assert( sqlite3_mutex_held(db->mutex) );

  /* For a long-term use prepared statement avoid the use of
  ** lookaside memory.
  */
  if( prepFlags & SQLITE_PREPARE_PERSISTENT ){
    sParse.disableLookaside++;
    db->lookaside.bDisable++;
  }
  sParse.disableVtab = (prepFlags & SQLITE_PREPARE_NO_VTAB)!=0;

  /* Check to verify that it is possible to get a read lock on all
  ** database schemas.  The inability to get a read lock indicates that
  ** some other database connection is holding a write-lock, which in
  ** turn means that the other connection has made uncommitted changes







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void sqlite3ParserReset(Parse *pParse){
  sqlite3 *db = pParse->db;
  sqlite3DbFree(db, pParse->aLabel);
  sqlite3ExprListDelete(db, pParse->pConstExpr);
  if( db ){
    assert( db->lookaside.bDisable >= pParse->disableLookaside );
    db->lookaside.bDisable -= pParse->disableLookaside;
    db->lookaside.sz = db->lookaside.bDisable ? 0 : db->lookaside.szTrue;
  }
  pParse->disableLookaside = 0;
}

/*
** Compile the UTF-8 encoded SQL statement zSql into a statement handle.
*/
................................................................................
  assert( sqlite3_mutex_held(db->mutex) );

  /* For a long-term use prepared statement avoid the use of
  ** lookaside memory.
  */
  if( prepFlags & SQLITE_PREPARE_PERSISTENT ){
    sParse.disableLookaside++;
    DisableLookaside;
  }
  sParse.disableVtab = (prepFlags & SQLITE_PREPARE_NO_VTAB)!=0;

  /* Check to verify that it is possible to get a read lock on all
  ** database schemas.  The inability to get a read lock indicates that
  ** some other database connection is holding a write-lock, which in
  ** turn means that the other connection has made uncommitted changes

Changes to src/sqliteInt.h.

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**
** Lookaside allocations are only allowed for objects that are associated
** with a particular database connection.  Hence, schema information cannot
** be stored in lookaside because in shared cache mode the schema information
** is shared by multiple database connections.  Therefore, while parsing
** schema information, the Lookaside.bEnabled flag is cleared so that
** lookaside allocations are not used to construct the schema objects.






*/
struct Lookaside {
  u32 bDisable;           /* Only operate the lookaside when zero */
  u16 sz;                 /* Size of each buffer in bytes */

  u8 bMalloced;           /* True if pStart obtained from sqlite3_malloc() */
  u32 nSlot;              /* Number of lookaside slots allocated */
  u32 anStat[3];          /* 0: hits.  1: size misses.  2: full misses */
  LookasideSlot *pInit;   /* List of buffers not previously used */
  LookasideSlot *pFree;   /* List of available buffers */
  void *pStart;           /* First byte of available memory space */
  void *pEnd;             /* First byte past end of available space */
};
struct LookasideSlot {
  LookasideSlot *pNext;    /* Next buffer in the list of free buffers */
};





/*
** A hash table for built-in function definitions.  (Application-defined
** functions use a regular table table from hash.h.)
**
** Hash each FuncDef structure into one of the FuncDefHash.a[] slots.
** Collisions are on the FuncDef.u.pHash chain.  Use the SQLITE_FUNC_HASH()







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>











>
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**
** Lookaside allocations are only allowed for objects that are associated
** with a particular database connection.  Hence, schema information cannot
** be stored in lookaside because in shared cache mode the schema information
** is shared by multiple database connections.  Therefore, while parsing
** schema information, the Lookaside.bEnabled flag is cleared so that
** lookaside allocations are not used to construct the schema objects.
**
** New lookaside allocations are only allowed if bDisable==0.  When
** bDisable is greater than zero, sz is set to zero which effectively
** disables lookaside without adding a new test for the bDisable flag
** in a performance-critical path.  sz should be set by to szTrue whenever
** bDisable changes back to zero.
*/
struct Lookaside {
  u32 bDisable;           /* Only operate the lookaside when zero */
  u16 sz;                 /* Size of each buffer in bytes */
  u16 szTrue;             /* True value of sz, even if disabled */
  u8 bMalloced;           /* True if pStart obtained from sqlite3_malloc() */
  u32 nSlot;              /* Number of lookaside slots allocated */
  u32 anStat[3];          /* 0: hits.  1: size misses.  2: full misses */
  LookasideSlot *pInit;   /* List of buffers not previously used */
  LookasideSlot *pFree;   /* List of available buffers */
  void *pStart;           /* First byte of available memory space */
  void *pEnd;             /* First byte past end of available space */
};
struct LookasideSlot {
  LookasideSlot *pNext;    /* Next buffer in the list of free buffers */
};

#define DisableLookaside  db->lookaside.bDisable++;db->lookaside.sz=0
#define EnableLookaside   db->lookaside.bDisable--;\
   db->lookaside.sz=db->lookaside.bDisable?0:db->lookaside.szTrue

/*
** A hash table for built-in function definitions.  (Application-defined
** functions use a regular table table from hash.h.)
**
** Hash each FuncDef structure into one of the FuncDefHash.a[] slots.
** Collisions are on the FuncDef.u.pHash chain.  Use the SQLITE_FUNC_HASH()