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Overview
Comment:Generalize the constant propagation optimization so that it applies on every WHERE close, not just those that contain a subquery. This then demonstrates that the current implementation is inadequate since it does not take into account collating sequences.
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SHA3-256: 57eb2abd5b270d65be5e0f138f0d46899fa6091df3ba20b0ea7ef04244a15d48
User & Date: drh 2018-07-26 23:47:11
Context
2018-07-26
23:54
Add a test case demonstrating the collation problem with constant propagation. check-in: 50add839 user: drh tags: propagate-const-opt
23:47
Generalize the constant propagation optimization so that it applies on every WHERE close, not just those that contain a subquery. This then demonstrates that the current implementation is inadequate since it does not take into account collating sequences. check-in: 57eb2abd user: drh tags: propagate-const-opt
21:16
Initial implementation of the WHERE-clause constant propagation optimization. check-in: 2fb82ad8 user: drh tags: propagate-const-opt
Changes
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Changes to src/expr.c.

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    return 2;
  }
  if( pA->op!=TK_COLUMN && pA->op!=TK_AGG_COLUMN && pA->u.zToken ){
    if( pA->op==TK_FUNCTION ){
      if( sqlite3StrICmp(pA->u.zToken,pB->u.zToken)!=0 ) return 2;
    }else if( pA->op==TK_COLLATE ){
      if( sqlite3_stricmp(pA->u.zToken,pB->u.zToken)!=0 ) return 2;
    }else if( strcmp(pA->u.zToken,pB->u.zToken)!=0 ){
      return 2;
    }
  }
  if( (pA->flags & EP_Distinct)!=(pB->flags & EP_Distinct) ) return 2;
  if( ALWAYS((combinedFlags & EP_TokenOnly)==0) ){
    if( combinedFlags & EP_xIsSelect ) return 2;
    if( sqlite3ExprCompare(pParse, pA->pLeft, pB->pLeft, iTab) ) return 2;







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    return 2;
  }
  if( pA->op!=TK_COLUMN && pA->op!=TK_AGG_COLUMN && pA->u.zToken ){
    if( pA->op==TK_FUNCTION ){
      if( sqlite3StrICmp(pA->u.zToken,pB->u.zToken)!=0 ) return 2;
    }else if( pA->op==TK_COLLATE ){
      if( sqlite3_stricmp(pA->u.zToken,pB->u.zToken)!=0 ) return 2;
    }else if( pA->op!=TK_UPLUS && strcmp(pA->u.zToken,pB->u.zToken)!=0 ){
      return 2;
    }
  }
  if( (pA->flags & EP_Distinct)!=(pB->flags & EP_Distinct) ) return 2;
  if( ALWAYS((combinedFlags & EP_TokenOnly)==0) ){
    if( combinedFlags & EP_xIsSelect ) return 2;
    if( sqlite3ExprCompare(pParse, pA->pLeft, pB->pLeft, iTab) ) return 2;

Changes to src/select.c.

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){
  pConst->nConst++;
  pConst->apExpr = sqlite3DbReallocOrFree(pConst->db, pConst->apExpr,
                         pConst->nConst*2*sizeof(Expr*));
  if( pConst->apExpr==0 ){
    pConst->nConst = 0;
  }else{

    pConst->apExpr[pConst->nConst*2-2] = pColumn;
    pConst->apExpr[pConst->nConst*2-1] = pValue;
  }
}

/*
** Find all instances of COLUMN=CONSTANT or CONSTANT=COLUMN in pExpr that
................................................................................
      sqlite3TreeViewSelect(0, p, 0);
    }
#endif
    if( p->pNext==0 ) ExplainQueryPlanPop(pParse);
    return rc;
  }
#endif















  /* For each term in the FROM clause, do two things:
  ** (1) Authorized unreferenced tables
  ** (2) Generate code for all sub-queries
  */
  for(i=0; i<pTabList->nSrc; i++){
    struct SrcList_item *pItem = &pTabList->a[i];
................................................................................
    ** may contain expression trees of at most
    ** (SQLITE_MAX_EXPR_DEPTH-Parse.nHeight) height. This is a bit
    ** more conservative than necessary, but much easier than enforcing
    ** an exact limit.
    */
    pParse->nHeight += sqlite3SelectExprHeight(p);

    /* Do the constant propagation optimization */
    if( OptimizationEnabled(db, SQLITE_PropagateConst)
     && propagateConstants(pParse, p)
    ){
#if SELECTTRACE_ENABLED
      if( sqlite3SelectTrace & 0x100 ){
        SELECTTRACE(0x100,pParse,p,("After constant propagation:\n"));
        sqlite3TreeViewSelect(0, p, 0);
      }
#endif
    }else{
      SELECTTRACE(0x100,pParse,p,("Constant propagation not possible\n"));
    }

    /* 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,
                           (pItem->fg.jointype & JT_OUTER)!=0)
    ){







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){
  pConst->nConst++;
  pConst->apExpr = sqlite3DbReallocOrFree(pConst->db, pConst->apExpr,
                         pConst->nConst*2*sizeof(Expr*));
  if( pConst->apExpr==0 ){
    pConst->nConst = 0;
  }else{
    while( pValue->op==TK_UPLUS ) pValue = pValue->pLeft;
    pConst->apExpr[pConst->nConst*2-2] = pColumn;
    pConst->apExpr[pConst->nConst*2-1] = pValue;
  }
}

/*
** Find all instances of COLUMN=CONSTANT or CONSTANT=COLUMN in pExpr that
................................................................................
      sqlite3TreeViewSelect(0, p, 0);
    }
#endif
    if( p->pNext==0 ) ExplainQueryPlanPop(pParse);
    return rc;
  }
#endif

  /* Do the constant propagation optimization */
  if( OptimizationEnabled(db, SQLITE_PropagateConst)
   && propagateConstants(pParse, p)
  ){
#if SELECTTRACE_ENABLED
    if( sqlite3SelectTrace & 0x100 ){
      SELECTTRACE(0x100,pParse,p,("After constant propagation:\n"));
      sqlite3TreeViewSelect(0, p, 0);
    }
#endif
  }else{
    SELECTTRACE(0x100,pParse,p,("Constant propagation not possible\n"));
  }

  /* For each term in the FROM clause, do two things:
  ** (1) Authorized unreferenced tables
  ** (2) Generate code for all sub-queries
  */
  for(i=0; i<pTabList->nSrc; i++){
    struct SrcList_item *pItem = &pTabList->a[i];
................................................................................
    ** may contain expression trees of at most
    ** (SQLITE_MAX_EXPR_DEPTH-Parse.nHeight) height. This is a bit
    ** more conservative than necessary, but much easier than enforcing
    ** 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,
                           (pItem->fg.jointype & JT_OUTER)!=0)
    ){

Changes to test/whereL.test.

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  |     |--LEFT-MOST SUBQUERY
  |     |  `--SEARCH TABLE t2 USING INDEX sqlite_autoindex_t2_1 (a=?)
  |     `--UNION ALL
  |        `--SEARCH TABLE t3 USING INDEX sqlite_autoindex_t3_1 (a=?)
  |--SCAN SUBQUERY xxxxxx
  `--SEARCH TABLE t1 USING INDEX sqlite_autoindex_t1_1 (a=?)
}
















finish_test








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  |     |--LEFT-MOST SUBQUERY
  |     |  `--SEARCH TABLE t2 USING INDEX sqlite_autoindex_t2_1 (a=?)
  |     `--UNION ALL
  |        `--SEARCH TABLE t3 USING INDEX sqlite_autoindex_t3_1 (a=?)
  |--SCAN SUBQUERY xxxxxx
  `--SEARCH TABLE t1 USING INDEX sqlite_autoindex_t1_1 (a=?)
}

# The scan of the t1 table goes first since that enables the ORDER BY
# sort to be omitted.  This would not be possible without constant
# propagation because without it the t1 table would depend on t3.
#
do_eqp_test 120 {
  SELECT * FROM t1, t2, t3
   WHERE t1.a=t2.a AND t2.a=t3.j AND t3.j=5
  ORDER BY t1.a;
} {
  QUERY PLAN
  |--SEARCH TABLE t1 USING INDEX sqlite_autoindex_t1_1 (a=?)
  |--SEARCH TABLE t2 USING INDEX sqlite_autoindex_t2_1 (a=?)
  `--SCAN TABLE t3
}

finish_test