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An initial attempt to optimize VIEWs that occur as the right operand of a

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LEFT JOIN.  This particular check-in does not work correctly because it does
not deal with the case of columns in the VIEW that return non-NULL even when
all columns in the table of the VIEW are NULL because of the LEFT JOIN.

FossilOrigin-Name: 1838a59c8a1c151bd6fc822b0ffef661803cf0e4704c917e74a04567b81740b9
This commit is contained in:
drh
2017-04-14 17:18:45 +00:00
parent b1ed717f18
commit 399c7e21e4
3 changed files with 41 additions and 33 deletions
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55
src/select.c
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@@ -3154,6 +3154,7 @@ static int multiSelectOrderBy(
typedef struct SubstContext {
Parse *pParse; /* The parsing context */
int iTable; /* Replace references to this table */
int iNewTable; /* New table number, or -1 */
ExprList *pEList; /* Replacement expressions */
} SubstContext;
@@ -3179,6 +3180,9 @@ static Expr *substExpr(
Expr *pExpr /* Expr in which substitution occurs */
){
if( pExpr==0 ) return 0;
if( ExprHasProperty(pExpr, EP_FromJoin) && pExpr->iRightJoinTable==pSubst->iTable ){
pExpr->iRightJoinTable = pSubst->iNewTable;
}
if( pExpr->op==TK_COLUMN && pExpr->iTable==pSubst->iTable ){
if( pExpr->iColumn<0 ){
pExpr->op = TK_NULL;
@@ -3285,7 +3289,13 @@ static void substSelect(
** due to ticket [2f7170d73bf9abf80] from 2015-02-09.)
**
** (3) The subquery is not the right operand of a left outer join
** (Originally ticket #306. Strengthened by ticket #3300)
** or (3b) the subquery is not a join and (3c) does not contain any
** result-set columns that could be non-NULL even if columns of the
** subquery table are NULL.
** to be more restrictive - disallowing subquery if it was the
** right operand of a left join. See tickets #306 and #3300. Relaxed
** to allow simple subqueries as the right operand of a left join
** on 2017-04-14.)
**
** (4) The subquery is not DISTINCT.
**
@@ -3403,6 +3413,8 @@ static int flattenSubquery(
SrcList *pSubSrc; /* The FROM clause of the subquery */
ExprList *pList; /* The result set of the outer query */
int iParent; /* VDBE cursor number of the pSub result set temp table */
int iNewParent = -1;/* Replacement table for iParent */
int isLeftJoin = 0; /* True if pSub is the right side of a LEFT JOIN */
int i; /* Loop counter */
Expr *pWhere; /* The WHERE clause */
struct SrcList_item *pSubitem; /* The subquery */
@@ -3429,7 +3441,7 @@ static int flattenSubquery(
return 0; /* Restriction (2b) */
}
}
pSubSrc = pSub->pSrc;
assert( pSubSrc );
/* Prior to version 3.1.2, when LIMIT and OFFSET had to be simple constants,
@@ -3467,10 +3479,9 @@ static int flattenSubquery(
return 0; /* Restriction (23) */
}
/* OBSOLETE COMMENT 1:
** Restriction 3: If the subquery is a join, make sure the subquery is
** not used as the right operand of an outer join. Examples of why this
** is not allowed:
/*
** If the subquery is the right operand of a LEFT JOIN, then the
** subquery may not be a join itself. Example of why this is not allowed:
**
** t1 LEFT OUTER JOIN (t2 JOIN t3)
**
@@ -3480,27 +3491,15 @@ static int flattenSubquery(
**
** which is not at all the same thing.
**
** OBSOLETE COMMENT 2:
** Restriction 12: If the subquery is the right operand of a left outer
** join, make sure the subquery has no WHERE clause.
** An examples of why this is not allowed:
**
** t1 LEFT OUTER JOIN (SELECT * FROM t2 WHERE t2.x>0)
**
** If we flatten the above, we would get
**
** (t1 LEFT OUTER JOIN t2) WHERE t2.x>0
**
** But the t2.x>0 test will always fail on a NULL row of t2, which
** effectively converts the OUTER JOIN into an INNER JOIN.
**
** THIS OVERRIDES OBSOLETE COMMENTS 1 AND 2 ABOVE:
** Ticket #3300 shows that flattening the right term of a LEFT JOIN
** is fraught with danger. Best to avoid the whole thing. If the
** subquery is the right term of a LEFT JOIN, then do not flatten.
** See also tickets #306 and #3300.
*/
if( (pSubitem->fg.jointype & JT_OUTER)!=0 ){
return 0;
isLeftJoin = 1;
if( pSubSrc->nSrc>1 ){
return 0; /* Restriction (3b) */
}
/* TBD: Deal with the case of result-set expressions that are non-NULL even
** when the RHS table of a LEFT JOIN is NULL. */
}
/* Restriction 17: If the sub-query is a compound SELECT, then it must
@@ -3709,6 +3708,7 @@ static int flattenSubquery(
sqlite3IdListDelete(db, pSrc->a[i+iFrom].pUsing);
assert( pSrc->a[i+iFrom].fg.isTabFunc==0 );
pSrc->a[i+iFrom] = pSubSrc->a[i];
iNewParent = pSubSrc->a[i].iCursor;
memset(&pSubSrc->a[i], 0, sizeof(pSubSrc->a[i]));
}
pSrc->a[iFrom].fg.jointype = jointype;
@@ -3754,6 +3754,9 @@ static int flattenSubquery(
pSub->pOrderBy = 0;
}
pWhere = sqlite3ExprDup(db, pSub->pWhere, 0);
if( isLeftJoin ){
setJoinExpr(pWhere, iNewParent);
}
if( subqueryIsAgg ){
assert( pParent->pHaving==0 );
pParent->pHaving = pParent->pWhere;
@@ -3770,6 +3773,7 @@ static int flattenSubquery(
SubstContext x;
x.pParse = pParse;
x.iTable = iParent;
x.iNewTable = iNewParent;
x.pEList = pSub->pEList;
substSelect(&x, pParent, 0);
}
@@ -3878,6 +3882,7 @@ static int pushDownWhereTerms(
pNew = sqlite3ExprDup(pParse->db, pWhere, 0);
x.pParse = pParse;
x.iTable = iCursor;
x.iNewTable = iCursor;
x.pEList = pSubq->pEList;
pNew = substExpr(&x, pNew);
pSubq->pWhere = sqlite3ExprAnd(pParse->db, pSubq->pWhere, pNew);