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Comment:Do not flatten subqueries that contain an ORDER BY or GROUP BY clause and can be implemented using a co-routine.
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SHA3-256: 042d655dd9002e8b89a798ad955b0285891aecf79f6978c5312e70ffe0609a46
User & Date: drh 2017-09-28 20:06:53
Context
2017-09-29
14:31
Merge changes from trunk. check-in: 06f432fb user: drh tags: prefer-coroutine-sort-subquery
2017-09-28
20:06
Do not flatten subqueries that contain an ORDER BY or GROUP BY clause and can be implemented using a co-routine. check-in: 042d655d user: drh tags: prefer-coroutine-sort-subquery
17:29
Remove the (undocumented) query-planner control that prevents a "SELECT ALL" subquery in FROM clause from being implemented as a co-routine. This control was added by [a29e117d7ec], where it was called a "stop-gap". check-in: ff2f5a31 user: drh tags: trunk
Changes
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Changes to src/select.c.

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    /* Catch mismatch in the declared columns of a view and the number of
    ** columns in the SELECT on the RHS */
    if( pTab->nCol!=pSub->pEList->nExpr ){
      sqlite3ErrorMsg(pParse, "expected %d columns for '%s' but got %d",
                      pTab->nCol, pTab->zName, pSub->pEList->nExpr);
      goto select_end;
    }




















    isAggSub = (pSub->selFlags & SF_Aggregate)!=0;
    if( flattenSubquery(pParse, p, i, isAgg, isAggSub) ){
      /* This subquery can be absorbed into its parent. */
      if( isAggSub ){
        isAgg = 1;
        p->selFlags |= SF_Aggregate;







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    /* Catch mismatch in the declared columns of a view and the number of
    ** columns in the SELECT on the RHS */
    if( pTab->nCol!=pSub->pEList->nExpr ){
      sqlite3ErrorMsg(pParse, "expected %d columns for '%s' but got %d",
                      pTab->nCol, pTab->zName, pSub->pEList->nExpr);
      goto select_end;
    }

    /* If the subquery contains an ORDER BY or GROUP BY clause and if
    ** it will be implemented as a co-routine, then do not flatten.  This
    ** restriction allows SQL constructs like this:
    **
    **  SELECT expensive_function(x)
    **    FROM (SELECT x FROM tab ORDER BY y LIMIT 10);
    **
    ** The expensive_function() is only computed on the 10 rows that
    ** are output, rather than every row of the table.
    */
    if( (pSub->pOrderBy!=0 || pSub->pGroupBy!=0)
     && i==0
     && (pTabList->nSrc==1
         || (pTabList->a[1].fg.jointype&(JT_LEFT|JT_CROSS))!=0)
     && OptimizationEnabled(db, SQLITE_SubqCoroutine)
    ){
      continue;
    }

    isAggSub = (pSub->selFlags & SF_Aggregate)!=0;
    if( flattenSubquery(pParse, p, i, isAgg, isAggSub) ){
      /* This subquery can be absorbed into its parent. */
      if( isAggSub ){
        isAgg = 1;
        p->selFlags |= SF_Aggregate;