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Overview
Comment:Avoid adding an artifical "LIMIT 1" on scalar subqueries that do not need it. This seems like a pointless optimization as it makes minimal run-time difference but does increase code complexity. Parked on a branch for historical reference.
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SHA3-256: c21628e9c166d8481138e04cfbc2ea5a54e92f6588d4407f615094e2e5a4b458
User & Date: drh 2017-06-17 19:06:17.565
Context
2017-06-17
19:06
Avoid adding an artifical "LIMIT 1" on scalar subqueries that do not need it. This seems like a pointless optimization as it makes minimal run-time difference but does increase code complexity. Parked on a branch for historical reference. (Leaf check-in: c21628e9c1 user: drh tags: subquery-limit-opt)
18:49
Fix a missing comma in the previous check-in. (check-in: f3db02f490 user: drh tags: trunk)
Changes
Unified Diff Ignore Whitespace Patch
Changes to src/expr.c.
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        VdbeComment((v, "Init subquery result"));
      }else{
        dest.eDest = SRT_Exists;
        sqlite3VdbeAddOp2(v, OP_Integer, 0, dest.iSDParm);
        VdbeComment((v, "Init EXISTS result"));
      }
      sqlite3ExprDelete(pParse->db, pSel->pLimit);



      pSel->pLimit = sqlite3ExprAlloc(pParse->db, TK_INTEGER,
                                  &sqlite3IntTokens[1], 0);

      pSel->iLimit = 0;
      pSel->selFlags &= ~SF_MultiValue;
      if( sqlite3Select(pParse, pSel, &dest) ){
        return 0;
      }
      rReg = dest.iSDParm;
      ExprSetVVAProperty(pExpr, EP_NoReduce);







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        VdbeComment((v, "Init subquery result"));
      }else{
        dest.eDest = SRT_Exists;
        sqlite3VdbeAddOp2(v, OP_Integer, 0, dest.iSDParm);
        VdbeComment((v, "Init EXISTS result"));
      }
      sqlite3ExprDelete(pParse->db, pSel->pLimit);
      if( (pSel->selFlags & SF_Aggregate)!=0 && pSel->pGroupBy==0 ){
        pSel->pLimit = 0;
      }else{
        pSel->pLimit = sqlite3ExprAlloc(pParse->db, TK_INTEGER,
                                    &sqlite3IntTokens[1], 0);
      }
      pSel->iLimit = 0;
      pSel->selFlags &= ~SF_MultiValue;
      if( sqlite3Select(pParse, pSel, &dest) ){
        return 0;
      }
      rReg = dest.iSDParm;
      ExprSetVVAProperty(pExpr, EP_NoReduce);