SQLite

  int nOBSat;           /* Number of ORDER BY terms satisfied by indices */
  int iECursor;         /* Cursor number for the sorter */
  int regReturn;        /* Register holding block-output return address */
  int labelBkOut;       /* Start label for the block-output subroutine */
  int addrSortIndex;    /* Address of the OP_SorterOpen or OP_OpenEphemeral */
  int labelDone;        /* Jump here when done, ex: LIMIT reached */
  u8 sortFlags;         /* Zero or more SORTFLAG_* bits */
  u8 bOrderedInnerLoop; /* ORDER BY correctly sorts the inner loop */
};
#define SORTFLAG_UseSorter  0x01   /* Use SorterOpen instead of OpenEphemeral */

/*
** Delete all the content of a Select structure.  Deallocate the structure
** itself only if bFree is true.
*/

    op = OP_SorterInsert;
  }else{
    op = OP_IdxInsert;
  }
  sqlite3VdbeAddOp2(v, op, pSort->iECursor, regRecord);
  if( iLimit ){
    int addr;
    int r1 = 0;
    /* Fill the sorter until it contains LIMIT+OFFSET entries.  (The iLimit
    ** register is initialized with value of LIMIT+OFFSET.)  After the sorter
    ** fills up, delete the least entry in the sorter after each insert.
    ** Thus we never hold more than the LIMIT+OFFSET rows in memory at once */
    addr = sqlite3VdbeAddOp3(v, OP_IfNotZero, iLimit, 0, 1); VdbeCoverage(v);
    sqlite3VdbeAddOp1(v, OP_Last, pSort->iECursor);
    if( pSort->bOrderedInnerLoop ){
      r1 = ++pParse->nMem;
      sqlite3VdbeAddOp3(v, OP_Column, pSort->iECursor, nExpr, r1);
      VdbeComment((v, "seq"));
    }
    sqlite3VdbeAddOp1(v, OP_Delete, pSort->iECursor);
    if( pSort->bOrderedInnerLoop ){
      /* If the inner loop is driven by an index such that values from
      ** the same iteration of the inner loop are in sorted order, then
      ** immediately jump to the next iteration of an inner loop if the
      ** entry from the current iteration does not fit into the top
      ** LIMIT+OFFSET entries of the sorter. */
      int iBrk = sqlite3VdbeCurrentAddr(v) + 2;
      sqlite3VdbeAddOp3(v, OP_Eq, regBase+nExpr, iBrk, r1);
      sqlite3VdbeChangeP5(v, SQLITE_NULLEQ);
      VdbeCoverage(v);
    }
    sqlite3VdbeJumpHere(v, addr);
  }
}

/*
** Add code to implement the OFFSET
*/

      p->nSelectRow = sqlite3WhereOutputRowCount(pWInfo);
    }
    if( sDistinct.isTnct && sqlite3WhereIsDistinct(pWInfo) ){
      sDistinct.eTnctType = sqlite3WhereIsDistinct(pWInfo);
    }
    if( sSort.pOrderBy ){
      sSort.nOBSat = sqlite3WhereIsOrdered(pWInfo);
      sSort.bOrderedInnerLoop = sqlite3WhereOrderedInnerLoop(pWInfo);
      if( sSort.nOBSat==sSort.pOrderBy->nExpr ){
        sSort.pOrderBy = 0;
      }
    }

    /* If sorting index that was created by a prior OP_OpenEphemeral 
    ** instruction ended up not being needed, then change the OP_OpenEphemeral

#define WHERE_OR_SUBCLAUSE     0x0020 /* Processing a sub-WHERE as part of
                                      ** the OR optimization  */
#define WHERE_GROUPBY          0x0040 /* pOrderBy is really a GROUP BY */
#define WHERE_DISTINCTBY       0x0080 /* pOrderby is really a DISTINCT clause */
#define WHERE_WANT_DISTINCT    0x0100 /* All output needs to be distinct */
#define WHERE_SORTBYGROUP      0x0200 /* Support sqlite3WhereIsSorted() */
#define WHERE_SEEK_TABLE       0x0400 /* Do not defer seeks on main table */
#define WHERE_ORDERBY_LIMIT    0x0800 /* ORDERBY+LIMIT on the inner loop */
                        /*     0x1000    not currently used */
                        /*     0x2000    not currently used */
#define WHERE_USE_LIMIT        0x4000 /* Use the LIMIT in cost estimates */
                        /*     0x8000    not currently used */

/* Allowed return values from sqlite3WhereIsDistinct()
*/

void sqlite3DeleteFrom(Parse*, SrcList*, Expr*);
void sqlite3Update(Parse*, SrcList*, ExprList*, Expr*, int);
WhereInfo *sqlite3WhereBegin(Parse*,SrcList*,Expr*,ExprList*,ExprList*,u16,int);
void sqlite3WhereEnd(WhereInfo*);
LogEst sqlite3WhereOutputRowCount(WhereInfo*);
int sqlite3WhereIsDistinct(WhereInfo*);
int sqlite3WhereIsOrdered(WhereInfo*);
int sqlite3WhereOrderedInnerLoop(WhereInfo*);
int sqlite3WhereIsSorted(WhereInfo*);
int sqlite3WhereContinueLabel(WhereInfo*);
int sqlite3WhereBreakLabel(WhereInfo*);
int sqlite3WhereOkOnePass(WhereInfo*, int*);
#define ONEPASS_OFF      0        /* Use of ONEPASS not allowed */
#define ONEPASS_SINGLE   1        /* ONEPASS valid for a single row update */
#define ONEPASS_MULTI    2        /* ONEPASS is valid for multiple rows */

/*
** Return TRUE if the WHERE clause returns rows in ORDER BY order.
** Return FALSE if the output needs to be sorted.
*/
int sqlite3WhereIsOrdered(WhereInfo *pWInfo){
  return pWInfo->nOBSat;
}

/*
** Return TRUE if the innermost loop of the WHERE clause implementation
** returns rows in ORDER BY order for complete run of the inner loop.
**
** Across multiple iterations of outer loops, the output rows need not be
** sorted.  As long as rows are sorted for just the innermost loop, this
** routine can return TRUE.
*/
int sqlite3WhereOrderedInnerLoop(WhereInfo *pWInfo){
  return pWInfo->bOrderedInnerLoop;
}

/*
** Return the VDBE address or label to jump to in order to continue
** immediately with the next row of a WHERE clause.
*/
int sqlite3WhereContinueLabel(WhereInfo *pWInfo){
  assert( pWInfo->iContinue!=0 );

** the pOrderBy terms can be matched in any order.  With ORDER BY, the 
** pOrderBy terms must be matched in strict left-to-right order.
*/
static i8 wherePathSatisfiesOrderBy(
  WhereInfo *pWInfo,    /* The WHERE clause */
  ExprList *pOrderBy,   /* ORDER BY or GROUP BY or DISTINCT clause to check */
  WherePath *pPath,     /* The WherePath to check */
  u16 wctrlFlags,       /* WHERE_GROUPBY or _DISTINCTBY or _ORDERBY_LIMIT */
  u16 nLoop,            /* Number of entries in pPath->aLoop[] */
  WhereLoop *pLast,     /* Add this WhereLoop to the end of pPath->aLoop[] */
  Bitmask *pRevMask     /* OUT: Mask of WhereLoops to run in reverse order */
){
  u8 revSet;            /* True if rev is known */
  u8 rev;               /* Composite sort order */
  u8 revIdx;            /* Index sort order */
  u8 isOrderDistinct;   /* All prior WhereLoops are order-distinct */
  u8 distinctColumns;   /* True if the loop has UNIQUE NOT NULL columns */
  u8 isMatch;           /* iColumn matches a term of the ORDER BY clause */
  u16 eqOpMask;         /* Allowed equality operators */
  u16 nKeyCol;          /* Number of key columns in pIndex */
  u16 nColumn;          /* Total number of ordered columns in the index */
  u16 nOrderBy;         /* Number terms in the ORDER BY clause */
  int iLoop;            /* Index of WhereLoop in pPath being processed */
  int i, j;             /* Loop counters */
  int iCur;             /* Cursor number for current WhereLoop */
  int iColumn;          /* A column number within table iCur */

  nOrderBy = pOrderBy->nExpr;
  testcase( nOrderBy==BMS-1 );
  if( nOrderBy>BMS-1 ) return 0;  /* Cannot optimize overly large ORDER BYs */
  isOrderDistinct = 1;
  obDone = MASKBIT(nOrderBy)-1;
  orderDistinctMask = 0;
  ready = 0;
  eqOpMask = WO_EQ | WO_IS | WO_ISNULL;
  if( wctrlFlags & WHERE_ORDERBY_LIMIT ) eqOpMask |= WO_IN;
  for(iLoop=0; isOrderDistinct && obSat<obDone && iLoop<=nLoop; iLoop++){
    if( iLoop>0 ) ready |= pLoop->maskSelf;
    if( iLoop<nLoop ){
      pLoop = pPath->aLoop[iLoop];
      if( wctrlFlags & WHERE_ORDERBY_LIMIT ) continue;
    }else{
      pLoop = pLast;
    }
    if( pLoop->wsFlags & WHERE_VIRTUALTABLE ){
      if( pLoop->u.vtab.isOrdered ) obSat = obDone;
      break;
    }
    iCur = pWInfo->pTabList->a[pLoop->iTab].iCursor;

    /* Mark off any ORDER BY term X that is a column in the table of
    ** the current loop for which there is term in the WHERE
    ** clause of the form X IS NULL or X=? that reference only outer
    ** loops.
    */
    for(i=0; i<nOrderBy; i++){
      if( MASKBIT(i) & obSat ) continue;
      pOBExpr = sqlite3ExprSkipCollate(pOrderBy->a[i].pExpr);
      if( pOBExpr->op!=TK_COLUMN ) continue;
      if( pOBExpr->iTable!=iCur ) continue;
      pTerm = sqlite3WhereFindTerm(&pWInfo->sWC, iCur, pOBExpr->iColumn,
                       ~ready, eqOpMask, 0);
      if( pTerm==0 ) continue;
      if( (pTerm->eOperator&(WO_EQ|WO_IS))!=0 && pOBExpr->iColumn>=0 ){
        const char *z1, *z2;
        pColl = sqlite3ExprCollSeq(pWInfo->pParse, pOrderBy->a[i].pExpr);
        if( !pColl ) pColl = db->pDfltColl;
        z1 = pColl->zName;
        pColl = sqlite3ExprCollSeq(pWInfo->pParse, pTerm->pExpr);

      ** that are not constrained by == or IN.
      */
      rev = revSet = 0;
      distinctColumns = 0;
      for(j=0; j<nColumn; j++){
        u8 bOnce;   /* True to run the ORDER BY search loop */

        /* Skip over == and IS and ISNULL terms.
        ** (Also skip IN terms when doing WHERE_ORDERBY_LIMIT processing)
        */
        if( j<pLoop->u.btree.nEq
         && pLoop->nSkip==0
         && ((i = pLoop->aLTerm[j]->eOperator) & eqOpMask)!=0
        ){
          if( i & WO_ISNULL ){
            testcase( isOrderDistinct );
            isOrderDistinct = 0;
          }
          continue;  
        }

  if( pWInfo->pOrderBy ){
    if( pWInfo->wctrlFlags & WHERE_DISTINCTBY ){
      if( pFrom->isOrdered==pWInfo->pOrderBy->nExpr ){
        pWInfo->eDistinct = WHERE_DISTINCT_ORDERED;
      }
    }else{
      pWInfo->nOBSat = pFrom->isOrdered;
      pWInfo->revMask = pFrom->revLoop;
      if( pWInfo->nOBSat<=0 ){
        pWInfo->nOBSat = 0;
        if( nLoop>0 ){
          Bitmask m;
          int rc = wherePathSatisfiesOrderBy(pWInfo, pWInfo->pOrderBy, pFrom,
                      WHERE_ORDERBY_LIMIT, nLoop-1, pFrom->aLoop[nLoop-1], &m);
          if( rc==pWInfo->pOrderBy->nExpr ){
            pWInfo->bOrderedInnerLoop = 1;
            pWInfo->revMask = m;
          }
        }
      }
    }
    if( (pWInfo->wctrlFlags & WHERE_SORTBYGROUP)
        && pWInfo->nOBSat==pWInfo->pOrderBy->nExpr && nLoop>0
    ){
      Bitmask revMask = 0;
      int nOrder = wherePathSatisfiesOrderBy(pWInfo, pWInfo->pOrderBy, 
          pFrom, 0, nLoop-1, pFrom->aLoop[nLoop-1], &revMask

  LogEst nRowOut;           /* Estimated number of output rows */
  LogEst iLimit;            /* LIMIT if wctrlFlags has WHERE_USE_LIMIT */
  u16 wctrlFlags;           /* Flags originally passed to sqlite3WhereBegin() */
  i8 nOBSat;                /* Number of ORDER BY terms satisfied by indices */
  u8 sorted;                /* True if really sorted (not just grouped) */
  u8 eOnePass;              /* ONEPASS_OFF, or _SINGLE, or _MULTI */
  u8 untestedTerms;         /* Not all WHERE terms resolved by outer loop */
  u8 eDistinct;             /* One of the WHERE_DISTINCT_* values */
  u8 nLevel;                /* Number of nested loop */
  u8 bOrderedInnerLoop;     /* True if only the inner-most loop is ordered */
  int iTop;                 /* The very beginning of the WHERE loop */
  int iContinue;            /* Jump here to continue with next record */
  int iBreak;               /* Jump here to break out of the loop */
  int savedNQueryLoop;      /* pParse->nQueryLoop outside the WHERE loop */
  int aiCurOnePass[2];      /* OP_OpenWrite cursors for the ONEPASS opt */
  WhereMaskSet sMaskSet;    /* Map cursor numbers to bitmasks */
  WhereClause sWC;          /* Decomposition of the WHERE clause */

# 2016-05-20
#
# 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 LIMIT in combination with ORDER BY
# and in particular, the optimizations in the inner loop that cause an
# early exit of the inner loop when the LIMIT is reached and the inner
# loop is emitting rows in ORDER BY order.


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

do_execsql_test limit2-100 {
  CREATE TABLE t1(a,b);
  WITH RECURSIVE c(x) AS (VALUES(1) UNION ALL SELECT x+1 FROM c WHERE x<1000)
    INSERT INTO t1(a,b) SELECT 1, (x*17)%1000 + 1000 FROM c;
  INSERT INTO t1(a,b) VALUES(2,2),(3,1006),(4,4),(5,9999);
  CREATE INDEX t1ab ON t1(a,b);
}
set sqlite_search_count 0
do_execsql_test limit2-100.1 {
  SELECT a, b, '|' FROM t1 WHERE a IN (2,4,5,3,1) ORDER BY b LIMIT 5;
} {2 2 | 4 4 | 1 1000 | 1 1001 | 1 1002 |}
set fast_count $sqlite_search_count
set sqlite_search_count 0
do_execsql_test limit2-100.2 {
  SELECT a, b, '|' FROM t1 WHERE a IN (2,4,5,3,1) ORDER BY +b LIMIT 5;
} {2 2 | 4 4 | 1 1000 | 1 1001 | 1 1002 |}
do_test limit2-100.3 {
  set slow_count $sqlite_search_count
  expr {$fast_count < 0.02*$slow_count}
} {1}

do_execsql_test limit2-110 {
  CREATE TABLE t2(x,y);
  INSERT INTO t2(x,y) VALUES('a',1),('a',2),('a',3),('a',4);
  INSERT INTO t2(x,y) VALUES('b',1),('c',2),('d',3),('e',4);
  CREATE INDEX t2xy ON t2(x,y);
}
set sqlite_search_count 0
do_execsql_test limit2-110.1 {
  SELECT a, b, '|' FROM t2, t1 WHERE t2.x='a' AND t1.a=t2.y ORDER BY t1.b LIMIT 5;
} {2 2 | 4 4 | 1 1000 | 1 1001 | 1 1002 |}
set fast_count $sqlite_search_count
set sqlite_search_count 0
do_execsql_test limit2-110.2 {
  SELECT a, b, '|' FROM t2, t1 WHERE t2.x='a' AND t1.a=t2.y ORDER BY +t1.b LIMIT 5;
} {2 2 | 4 4 | 1 1000 | 1 1001 | 1 1002 |}
set slow_count $sqlite_search_count
do_test limit2-110.3 {
  expr {$fast_count < 0.02*$slow_count}
} {1}

do_execsql_test limit2-120 {
  DROP INDEX t1ab;
  CREATE INDEX t1ab ON t1(a,b DESC);
}
set sqlite_search_count 0
do_execsql_test limit2-120.1 {
  SELECT a, b, '|' FROM t1 WHERE a IN (2,4,5,3,1) ORDER BY b DESC LIMIT 5;
} {5 9999 | 1 1999 | 1 1998 | 1 1997 | 1 1996 |}
set fast_count $sqlite_search_count
set sqlite_search_count 0
do_execsql_test limit2-120.2 {
  SELECT a, b, '|' FROM t1 WHERE a IN (2,4,5,3,1) ORDER BY +b DESC LIMIT 5;
} {5 9999 | 1 1999 | 1 1998 | 1 1997 | 1 1996 |}
do_test limit2-120.3 {
  set slow_count $sqlite_search_count
  expr {$fast_count < 0.02*$slow_count}
} {1}



finish_test