SQLite

    v = sqlite3GetVdbe(pParse);
    if( NEVER(v==0) ) return;  /* VDBE should have already been allocated */
    if( sqlite3ExprIsInteger(p->pLimit, &n) ){
      sqlite3VdbeAddOp2(v, OP_Integer, n, iLimit);
      VdbeComment((v, "LIMIT counter"));
      if( n==0 ){
        sqlite3VdbeAddOp2(v, OP_Goto, 0, iBreak);
      }else{
        if( p->nSelectRow > (double)n ) p->nSelectRow = (double)n;
      }
    }else{
      sqlite3ExprCode(pParse, p->pLimit, iLimit);
      sqlite3VdbeAddOp1(v, OP_MustBeInt, iLimit);
      VdbeComment((v, "LIMIT counter"));
      sqlite3VdbeAddOp2(v, OP_IfZero, iLimit, iBreak);
    }

  }

  /* Generate code for the left and right SELECT statements.
  */
  switch( p->op ){
    case TK_ALL: {
      int addr = 0;
      int nLimit;
      assert( !pPrior->pLimit );
      pPrior->pLimit = p->pLimit;
      pPrior->pOffset = p->pOffset;
      explainSetInteger(iSub1, pParse->iNextSelectId);
      rc = sqlite3Select(pParse, pPrior, &dest);
      p->pLimit = 0;
      p->pOffset = 0;

        VdbeComment((v, "Jump ahead if LIMIT reached"));
      }
      explainSetInteger(iSub2, pParse->iNextSelectId);
      rc = sqlite3Select(pParse, p, &dest);
      testcase( rc!=SQLITE_OK );
      pDelete = p->pPrior;
      p->pPrior = pPrior;
      p->nSelectRow += pPrior->nSelectRow;
      if( pPrior->pLimit
       && sqlite3ExprIsInteger(pPrior->pLimit, &nLimit)
       && p->nSelectRow > (double)nLimit 
      ){
        p->nSelectRow = (double)nLimit;
      }
      if( addr ){
        sqlite3VdbeJumpHere(v, addr);
      }
      break;
    }
    case TK_EXCEPT:
    case TK_UNION: {

      testcase( rc!=SQLITE_OK );
      /* Query flattening in sqlite3Select() might refill p->pOrderBy.
      ** Be sure to delete p->pOrderBy, therefore, to avoid a memory leak. */
      sqlite3ExprListDelete(db, p->pOrderBy);
      pDelete = p->pPrior;
      p->pPrior = pPrior;
      p->pOrderBy = 0;
      if( p->op==TK_UNION ) p->nSelectRow += pPrior->nSelectRow;
      sqlite3ExprDelete(db, p->pLimit);
      p->pLimit = pLimit;
      p->pOffset = pOffset;
      p->iLimit = 0;
      p->iOffset = 0;

      /* Convert the data in the temporary table into whatever form

      p->pOffset = 0;
      intersectdest.iParm = tab2;
      explainSetInteger(iSub2, pParse->iNextSelectId);
      rc = sqlite3Select(pParse, p, &intersectdest);
      testcase( rc!=SQLITE_OK );
      pDelete = p->pPrior;
      p->pPrior = pPrior;
      if( p->nSelectRow>pPrior->nSelectRow ) p->nSelectRow = pPrior->nSelectRow;
      sqlite3ExprDelete(db, p->pLimit);
      p->pLimit = pLimit;
      p->pOffset = pOffset;

      /* Generate code to take the intersection of the two temporary
      ** tables.
      */

  if( op==TK_EXCEPT || op==TK_INTERSECT ){
    addrEofA = sqlite3VdbeAddOp2(v, OP_Goto, 0, labelEnd);
  }else{  
    addrEofA = sqlite3VdbeAddOp2(v, OP_If, regEofB, labelEnd);
    sqlite3VdbeAddOp2(v, OP_Gosub, regOutB, addrOutB);
    sqlite3VdbeAddOp1(v, OP_Yield, regAddrB);
    sqlite3VdbeAddOp2(v, OP_Goto, 0, addrEofA);
    p->nSelectRow += pPrior->nSelectRow;
  }

  /* Generate a subroutine to run when the results from select B
  ** are exhausted and only data in select A remains.
  */
  if( op==TK_INTERSECT ){
    addrEofB = addrEofA;
    if( p->nSelectRow > pPrior->nSelectRow ) p->nSelectRow = pPrior->nSelectRow;
  }else{  
    VdbeNoopComment((v, "eof-B subroutine"));
    addrEofB = sqlite3VdbeAddOp2(v, OP_If, regEofA, labelEnd);
    sqlite3VdbeAddOp2(v, OP_Gosub, regOutA, addrOutA);
    sqlite3VdbeAddOp1(v, OP_Yield, regAddrA);
    sqlite3VdbeAddOp2(v, OP_Goto, 0, addrEofB);
  }

      i = -1;
    }else{
      sqlite3SelectDestInit(&dest, SRT_EphemTab, pItem->iCursor);
      assert( pItem->isPopulated==0 );
      explainSetInteger(pItem->iSelectId, pParse->iNextSelectId);
      sqlite3Select(pParse, pSub, &dest);
      pItem->isPopulated = 1;
      pItem->pTab->nRowEst = (unsigned)pSub->nSelectRow;
    }
    if( /*pParse->nErr ||*/ db->mallocFailed ){
      goto select_end;
    }
    pParse->nHeight -= sqlite3SelectExprHeight(p);
    pTabList = p->pSrc;
    if( !IgnorableOrderby(pDest) ){

  if( pDest->eDest==SRT_EphemTab ){
    sqlite3VdbeAddOp2(v, OP_OpenEphemeral, pDest->iParm, pEList->nExpr);
  }

  /* Set the limiter.
  */
  iEnd = sqlite3VdbeMakeLabel(v);
  p->nSelectRow = (double)LARGEST_INT64;
  computeLimitRegisters(pParse, p, iEnd);

  /* Open a virtual index to use for the distinct set.
  */
  if( p->selFlags & SF_Distinct ){
    KeyInfo *pKeyInfo;
    assert( isAgg || pGroupBy );

  /* Aggregate and non-aggregate queries are handled differently */
  if( !isAgg && pGroupBy==0 ){
    /* This case is for non-aggregate queries
    ** Begin the database scan
    */
    pWInfo = sqlite3WhereBegin(pParse, pTabList, pWhere, &pOrderBy, 0);
    if( pWInfo==0 ) goto select_end;
    if( pWInfo->nRowOut < p->nSelectRow ) p->nSelectRow = pWInfo->nRowOut;

    /* If sorting index that was created by a prior OP_OpenEphemeral 
    ** instruction ended up not being needed, then change the OP_OpenEphemeral
    ** into an OP_Noop.
    */
    if( addrSortIndex>=0 && pOrderBy==0 ){
      sqlite3VdbeChangeToNoop(v, addrSortIndex, 1);

      for(k=p->pEList->nExpr, pItem=p->pEList->a; k>0; k--, pItem++){
        pItem->iAlias = 0;
      }
      for(k=pGroupBy->nExpr, pItem=pGroupBy->a; k>0; k--, pItem++){
        pItem->iAlias = 0;
      }
      if( p->nSelectRow>(double)100 ) p->nSelectRow = (double)100;
    }else{
      p->nSelectRow = (double)1;
    }

 
    /* Create a label to jump to when we want to abort the query */
    addrEnd = sqlite3VdbeMakeLabel(v);

    /* Convert TK_COLUMN nodes into TK_AGG_COLUMN and make entries in

  SrcList *pTabList;             /* List of tables in the join */
  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 nLevel;                    /* Number of nested loop */
  struct WhereClause *pWC;       /* Decomposition of the WHERE clause */
  double savedNQueryLoop;        /* pParse->nQueryLoop outside the WHERE loop */
  double nRowOut;                /* Estimated number of output rows */
  WhereLevel a[1];               /* Information about each nest loop in WHERE */
};

/*
** A NameContext defines a context in which to resolve table and column
** names.  The context consists of a list of tables (the pSrcList) field and
** a list of named expression (pEList).  The named expression list may

  Select *pPrior;        /* Prior select in a compound select statement */
  Select *pNext;         /* Next select to the left in a compound */
  Select *pRightmost;    /* Right-most select in a compound select statement */
  Expr *pLimit;          /* LIMIT expression. NULL means not used. */
  Expr *pOffset;         /* OFFSET expression. NULL means not used. */
  int iLimit, iOffset;   /* Memory registers holding LIMIT & OFFSET counters */
  int addrOpenEphm[3];   /* OP_OpenEphem opcodes related to this select */
  double nSelectRow;     /* Estimated number of result rows */
};

/*
** Allowed values for Select.selFlags.  The "SF" prefix stands for
** "Select Flag".
*/
#define SF_Distinct        0x0001  /* Output should be DISTINCT */

  }

  /* Open all tables in the pTabList and any indices selected for
  ** searching those tables.
  */
  sqlite3CodeVerifySchema(pParse, -1); /* Insert the cookie verifier Goto */
  notReady = ~(Bitmask)0;
  pWInfo->nRowOut = (double)1;
  for(i=0, pLevel=pWInfo->a; i<nTabList; i++, pLevel++){
    Table *pTab;     /* Table to open */
    int iDb;         /* Index of database containing table/index */

    pTabItem = &pTabList->a[pLevel->iFrom];
    pTab = pTabItem->pTab;
    pLevel->iTabCur = pTabItem->iCursor;
    pWInfo->nRowOut *= pLevel->plan.nRow;
    iDb = sqlite3SchemaToIndex(db, pTab->pSchema);
    if( (pTab->tabFlags & TF_Ephemeral)!=0 || pTab->pSelect ){
      /* Do nothing */
    }else
#ifndef SQLITE_OMIT_VIRTUALTABLE
    if( (pLevel->plan.wsFlags & WHERE_VIRTUALTABLE)!=0 ){
      const char *pVTab = (const char *)sqlite3GetVTable(db, pTab);

   ORDER BY x.registering_flock;
} {
  1 0 0 {SCAN TABLE sheep AS s (~1000000 rows)} 
  1 1 1 {SEARCH TABLE flock_owner AS prev USING INDEX sqlite_autoindex_flock_owner_1 (flock_no=? AND owner_change_date<?) (~2 rows)} 
  1 0 0 {EXECUTE CORRELATED SCALAR SUBQUERY 2} 
  2 0 0 {SEARCH TABLE flock_owner AS later USING COVERING INDEX sqlite_autoindex_flock_owner_1 (flock_no=? AND owner_change_date>? AND owner_change_date<?) (~1 rows)} 
  0 0 0 {SCAN TABLE sheep AS x USING INDEX sheep_reg_flock_index (~1000000 rows)} 
  0 1 1 {SEARCH SUBQUERY 1 AS y USING AUTOMATIC COVERING INDEX (sheep_no=?) (~7 rows)}
  0 1 1 {SEARCH SUBQUERY 1 AS y USING AUTOMATIC COVERING INDEX (sheep_no=?) (~8 rows)}
}

finish_test

do_eqp_test 1.6 {
  SELECT DISTINCT count(*) FROM t3 GROUP BY a;
} {
  0 0 0 {SCAN TABLE t3 (~1000000 rows)}
  0 0 0 {USE TEMP B-TREE FOR GROUP BY}
  0 0 0 {USE TEMP B-TREE FOR DISTINCT}
}

do_eqp_test 1.7 {
  SELECT * FROM t3 JOIN (SELECT 1)
} {
  0 0 1 {SCAN SUBQUERY 1 (~1 rows)}
  0 1 0 {SCAN TABLE t3 (~1000000 rows)}
}
do_eqp_test 1.8 {
  SELECT * FROM t3 JOIN (SELECT 1 UNION SELECT 2)
} {
  1 0 0 {COMPOUND SUBQUERIES 2 AND 3 USING TEMP B-TREE (UNION)}
  0 0 1 {SCAN SUBQUERY 1 (~2 rows)}
  0 1 0 {SCAN TABLE t3 (~1000000 rows)}
}
do_eqp_test 1.9 {
  SELECT * FROM t3 JOIN (SELECT 1 EXCEPT SELECT a FROM t3 LIMIT 17)
} {
  3 0 0 {SCAN TABLE t3 (~1000000 rows)}
  1 0 0 {COMPOUND SUBQUERIES 2 AND 3 USING TEMP B-TREE (EXCEPT)}
  0 0 1 {SCAN SUBQUERY 1 (~17 rows)}
  0 1 0 {SCAN TABLE t3 (~1000000 rows)}
}
do_eqp_test 1.10 {
  SELECT * FROM t3 JOIN (SELECT 1 INTERSECT SELECT a FROM t3 LIMIT 17)
} {
  3 0 0 {SCAN TABLE t3 (~1000000 rows)}
  1 0 0 {COMPOUND SUBQUERIES 2 AND 3 USING TEMP B-TREE (INTERSECT)}
  0 0 1 {SCAN SUBQUERY 1 (~1 rows)}
  0 1 0 {SCAN TABLE t3 (~1000000 rows)}
}

do_eqp_test 1.11 {
  SELECT * FROM t3 JOIN (SELECT 1 UNION ALL SELECT a FROM t3 LIMIT 17)
} {
  3 0 0 {SCAN TABLE t3 (~1000000 rows)}
  1 0 0 {COMPOUND SUBQUERIES 2 AND 3 (UNION ALL)}
  0 0 1 {SCAN SUBQUERY 1 (~17 rows)}
  0 1 0 {SCAN TABLE t3 (~1000000 rows)}
}

#-------------------------------------------------------------------------
# Test cases eqp-2.* - tests for single select statements.
#
drop_all_tables
do_execsql_test 2.1 {
  CREATE TABLE t1(x, y);

}

det 3.2.1 {
  SELECT * FROM (SELECT * FROM t1 ORDER BY x LIMIT 10) ORDER BY y LIMIT 5
} {
  1 0 0 {SCAN TABLE t1 (~1000000 rows)} 
  1 0 0 {USE TEMP B-TREE FOR ORDER BY} 
  0 0 0 {SCAN SUBQUERY 1 (~1000000 rows)} 
  0 0 0 {SCAN SUBQUERY 1 (~10 rows)} 
  0 0 0 {USE TEMP B-TREE FOR ORDER BY}
}
det 3.2.2 {
  SELECT * FROM 
    (SELECT * FROM t1 ORDER BY x LIMIT 10) AS x1,
    (SELECT * FROM t2 ORDER BY x LIMIT 10) AS x2
  ORDER BY x2.y LIMIT 5
} {
  1 0 0 {SCAN TABLE t1 (~1000000 rows)} 
  1 0 0 {USE TEMP B-TREE FOR ORDER BY} 
  2 0 0 {SCAN TABLE t2 USING INDEX t2i1 (~1000000 rows)} 
  0 0 0 {SCAN SUBQUERY 1 AS x1 (~1000000 rows)} 
  0 1 1 {SCAN SUBQUERY 2 AS x2 (~1000000 rows)} 
  0 0 0 {SCAN SUBQUERY 1 AS x1 (~10 rows)} 
  0 1 1 {SCAN SUBQUERY 2 AS x2 (~10 rows)} 
  0 0 0 {USE TEMP B-TREE FOR ORDER BY}
}

det 3.3.1 {
  SELECT * FROM t1 WHERE y IN (SELECT y FROM t2)
} {
  0 0 0 {SCAN TABLE t1 (~100000 rows)} 

# count(*) FROM (SELECT max(b) AS x FROM t1 GROUP BY a) GROUP BY x;
# 1|0|0|SCAN TABLE t1 USING COVERING INDEX i2 (~1000000 rows) 0|0|0|SCAN
# SUBQUERY 1 (~1000000 rows) 0|0|0|USE TEMP B-TREE FOR GROUP BY
det 5.10 {
  SELECT count(*) FROM (SELECT max(b) AS x FROM t1 GROUP BY a) GROUP BY x
} {
  1 0 0 {SCAN TABLE t1 USING COVERING INDEX i2 (~1000000 rows)}
  0 0 0 {SCAN SUBQUERY 1 (~1000000 rows)}
  0 0 0 {SCAN SUBQUERY 1 (~100 rows)}
  0 0 0 {USE TEMP B-TREE FOR GROUP BY}
}

# EVIDENCE-OF: R-18544-33103 sqlite> EXPLAIN QUERY PLAN SELECT * FROM
# (SELECT * FROM t2 WHERE c=1), t1; 0|0|0|SEARCH TABLE t2 USING INDEX i4
# (c=?) (~10 rows) 0|1|1|SCAN TABLE t1 (~1000000 rows)
det 5.11 "SELECT * FROM (SELECT * FROM t2 WHERE c=1), t1" {