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Overview
Comment:Relax LEFT-JOIN restrictions on the push-down optimization.
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Timelines: family | ancestors | descendants | both | join-strength-reduction
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SHA3-256: b5d3dd8cb0b1e42ed0671a12d22af05194ea9522e4f41fd4bb0deff70b8b0757
User & Date: drh 2018-03-21 01:59:46
References
2018-03-27
15:13
The push-down optimization was being too aggressive such that it sometimes generated incorrect results. Reinstate the restriction (4) (with qualifications) that was removed by check-ins [b5d3dd8cb0b1e4] and [dd568c27b1d765]. check-in: f08c1731 user: drh tags: trunk
Context
2018-03-22
12:00
Add the left join strength reduction optimization. Enhance the push-down optimization so that it works with many LEFT JOINs. check-in: dd568c27 user: drh tags: trunk
2018-03-21
01:59
Relax LEFT-JOIN restrictions on the push-down optimization. Closed-Leaf check-in: b5d3dd8c user: drh tags: join-strength-reduction
2018-03-20
22:52
Do a more thorough job of cleaning traces of the strength-reduced LEFT JOIN. check-in: 08833dda user: drh tags: join-strength-reduction
Changes
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Changes to src/select.c.

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** term that is marked with EP_FromJoin and iRightJoinTable==iTable into
** an ordinary term that omits the EP_FromJoin mark.
**
** This happens when a LEFT JOIN is simplified into an ordinary JOIN.
*/
static void unsetJoinExpr(Expr *p, int iTable){
  while( p ){
    if( ExprHasProperty(p, EP_FromJoin) && p->iRightJoinTable==iTable ){

      ExprClearProperty(p, EP_FromJoin);
    }
    if( p->op==TK_FUNCTION && p->x.pList ){
      int i;
      for(i=0; i<p->x.pList->nExpr; i++){
        unsetJoinExpr(p->x.pList->a[i].pExpr, iTable);
      }
................................................................................
**           to suppress it. **)
**
**   (2) The inner query is the recursive part of a common table expression.
**
**   (3) The inner query has a LIMIT clause (since the changes to the WHERE
**       close would change the meaning of the LIMIT).
**

**   (4) The inner query is the right operand of a LEFT JOIN.  (The caller
**       enforces this restriction since this routine does not have enough
**       information to know.)
**
**   (5) The WHERE clause expression originates in the ON or USING clause
**       of a LEFT JOIN.










**
** Return 0 if no changes are made and non-zero if one or more WHERE clause
** terms are duplicated into the subquery.
*/
static int pushDownWhereTerms(
  Parse *pParse,        /* Parse context (for malloc() and error reporting) */
  Select *pSubq,        /* The subquery whose WHERE clause is to be augmented */
................................................................................
  if( pSubq->pLimit!=0 ){
    return 0; /* restriction (3) */
  }
  while( pWhere->op==TK_AND ){
    nChng += pushDownWhereTerms(pParse, pSubq, pWhere->pRight, iCursor);
    pWhere = pWhere->pLeft;
  }

  if( ExprHasProperty(pWhere,EP_FromJoin) ) return 0; /* restriction (5) */

  if( sqlite3ExprIsTableConstant(pWhere, iCursor) ){
    nChng++;
    while( pSubq ){
      SubstContext x;
      pNew = sqlite3ExprDup(pParse->db, pWhere, 0);

      x.pParse = pParse;
      x.iTable = iCursor;
      x.iNewTable = iCursor;
      x.isLeftJoin = 0;
      x.pEList = pSubq->pEList;
      pNew = substExpr(&x, pNew);
      if( pSubq->selFlags & SF_Aggregate ){
................................................................................
    ** an exact limit.
    */
    pParse->nHeight += sqlite3SelectExprHeight(p);

    /* Make copies of constant WHERE-clause terms in the outer query down
    ** inside the subquery.  This can help the subquery to run more efficiently.
    */
    if( (pItem->fg.jointype & JT_OUTER)==0
     && OptimizationEnabled(db, SQLITE_PushDown)
     && pushDownWhereTerms(pParse, pSub, p->pWhere, pItem->iCursor)
    ){
#if SELECTTRACE_ENABLED
      if( sqlite3SelectTrace & 0x100 ){
        SELECTTRACE(0x100,pParse,p,("After WHERE-clause push-down:\n"));
        sqlite3TreeViewSelect(0, p, 0);
      }







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** term that is marked with EP_FromJoin and iRightJoinTable==iTable into
** an ordinary term that omits the EP_FromJoin mark.
**
** This happens when a LEFT JOIN is simplified into an ordinary JOIN.
*/
static void unsetJoinExpr(Expr *p, int iTable){
  while( p ){
    if( ExprHasProperty(p, EP_FromJoin)
     && (iTable<0 || p->iRightJoinTable==iTable) ){
      ExprClearProperty(p, EP_FromJoin);
    }
    if( p->op==TK_FUNCTION && p->x.pList ){
      int i;
      for(i=0; i<p->x.pList->nExpr; i++){
        unsetJoinExpr(p->x.pList->a[i].pExpr, iTable);
      }
................................................................................
**           to suppress it. **)
**
**   (2) The inner query is the recursive part of a common table expression.
**
**   (3) The inner query has a LIMIT clause (since the changes to the WHERE
**       close would change the meaning of the LIMIT).
**
**   (4) (** This restriction was removed on 2018-03-21.  It used to read:
**       The inner query is the right operand of a LEFT JOIN. **)


**
**   (5) The WHERE clause expression originates in the ON or USING clause
**       of a LEFT JOIN where iCursor is not the right-hand table of that
**       left join.  An example:
**
**           SELECT *
**           FROM (SELECT 1 AS a1 UNION ALL SELECT 2) AS aa
**           JOIN (SELECT 1 AS b2 UNION ALL SELECT 2) AS bb ON (a1=b2)
**           LEFT JOIN (SELECT 8 AS c3 UNION ALL SELECT 9) AS cc ON (b2=2);
**
**       The correct answer is three rows:  (1,1,NULL),(2,2,8),(2,2,9).
**       But if the (b2=2) term were to be pushed down into the bb subquery,
**       then the (1,1,NULL) row would be suppressed.
**
** Return 0 if no changes are made and non-zero if one or more WHERE clause
** terms are duplicated into the subquery.
*/
static int pushDownWhereTerms(
  Parse *pParse,        /* Parse context (for malloc() and error reporting) */
  Select *pSubq,        /* The subquery whose WHERE clause is to be augmented */
................................................................................
  if( pSubq->pLimit!=0 ){
    return 0; /* restriction (3) */
  }
  while( pWhere->op==TK_AND ){
    nChng += pushDownWhereTerms(pParse, pSubq, pWhere->pRight, iCursor);
    pWhere = pWhere->pLeft;
  }
  if( ExprHasProperty(pWhere,EP_FromJoin) && pWhere->iRightJoinTable!=iCursor ){
    return 0; /* restriction (5) */
  }
  if( sqlite3ExprIsTableConstant(pWhere, iCursor) ){
    nChng++;
    while( pSubq ){
      SubstContext x;
      pNew = sqlite3ExprDup(pParse->db, pWhere, 0);
      unsetJoinExpr(pNew, -1);
      x.pParse = pParse;
      x.iTable = iCursor;
      x.iNewTable = iCursor;
      x.isLeftJoin = 0;
      x.pEList = pSubq->pEList;
      pNew = substExpr(&x, pNew);
      if( pSubq->selFlags & SF_Aggregate ){
................................................................................
    ** an exact limit.
    */
    pParse->nHeight += sqlite3SelectExprHeight(p);

    /* Make copies of constant WHERE-clause terms in the outer query down
    ** inside the subquery.  This can help the subquery to run more efficiently.
    */

    if( OptimizationEnabled(db, SQLITE_PushDown)
     && pushDownWhereTerms(pParse, pSub, p->pWhere, pItem->iCursor)
    ){
#if SELECTTRACE_ENABLED
      if( sqlite3SelectTrace & 0x100 ){
        SELECTTRACE(0x100,pParse,p,("After WHERE-clause push-down:\n"));
        sqlite3TreeViewSelect(0, p, 0);
      }