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Include WHERE terms in the cursor-hint passed to a cursor opened for the rhs of a LEFT JOIN iff we can be sure that those terms will not evaluate to true if the LEFT JOIN generates a row of NULLs.

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FossilOrigin-Name: 998095aba01b75f685ed981b377e1dfe650d9bbf
This commit is contained in:
dan
2016-06-17 19:27:13 +00:00
parent c497441406
commit e6912fd819
4 changed files with 122 additions and 19 deletions
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56
src/wherecode.c
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@ -626,6 +626,31 @@ static int codeCursorHintCheckExpr(Walker *pWalker, Expr *pExpr){
return WRC_Continue;
}
/*
** Test whether or not expression pExpr, which was part of a WHERE clause,
** should be included in the cursor-hint for a table that is on the rhs
** of a LEFT JOIN. Set Walker.eCode to non-zero before returning if the
** expression is not suitable.
**
** An expression is unsuitable if it might evaluate to non NULL even if
** a TK_COLUMN node that does affect the value of the expression is set
** to NULL. For example:
**
** col IS NULL
** col IS NOT NULL
** coalesce(col, 1)
** CASE WHEN col THEN 0 ELSE 1 END
*/
static int codeCursorHintIsOrFunction(Walker *pWalker, Expr *pExpr){
if( pExpr->op==TK_IS || pExpr->op==TK_FUNCTION
|| pExpr->op==TK_ISNULL || pExpr->op==TK_ISNOT
|| pExpr->op==TK_NOTNULL || pExpr->op==TK_CASE
){
pWalker->eCode = 1;
}
return WRC_Continue;
}
/*
** This function is called on every node of an expression tree used as an
@ -714,22 +739,33 @@ static void codeCursorHint(
** JOIN for which the current table is not the rhs are omitted
** from the cursor-hint.
**
** If this table is the rhs of a LEFT JOIN, only terms that were
** specified as part of the ON(...) clause may be included in the
** hint. This is to address the following:
** If this table is the rhs of a LEFT JOIN, "IS" or "IS NULL" terms
** that were specified as part of the WHERE clause must be excluded.
** This is to address the following:
**
** SELECT ... t1 LEFT JOIN t2 ON (t1.a=t2.b) WHERE t2.c IS NULL;
**
** If the (t2.c IS NULL) constraint is pushed down to the cursor, it
** might filter out all rows that match (t1.a=t2.b), causing SQLite
** to add a row of NULL values to the output that should not be
** present (since the ON clause does actually match rows within t2).
** Say there is a single row in t2 that matches (t1.a=t2.b), but its
** t2.c values is not NULL. If the (t2.c IS NULL) constraint is
** pushed down to the cursor, this row is filtered out, causing
** SQLite to synthesize a row of NULL values. Which does match the
** WHERE clause, and so the query returns a row. Which is incorrect.
**
** For the same reason, WHERE terms such as:
**
** WHERE 1 = (t2.c IS NULL)
**
** are also excluded. See codeCursorHintIsOrFunction() for details.
*/
if( pTabItem->fg.jointype & JT_LEFT ){
if( !ExprHasProperty(pTerm->pExpr, EP_FromJoin)
|| pTerm->pExpr->iRightJoinTable!=pTabItem->iCursor
Expr *pExpr = pTerm->pExpr;
if( !ExprHasProperty(pExpr, EP_FromJoin)
|| pExpr->iRightJoinTable!=pTabItem->iCursor
){
continue;
sWalker.eCode = 0;
sWalker.xExprCallback = codeCursorHintIsOrFunction;
sqlite3WalkExpr(&sWalker, pTerm->pExpr);
if( sWalker.eCode ) continue;
}
}else{
if( ExprHasProperty(pTerm->pExpr, EP_FromJoin) ) continue;