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Add testcase() macros and improve comments in the LEFT JOIN strength reduction

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optimization.

FossilOrigin-Name: 5613457714dd74d6da8e387132a0d8e64980ba4a921a9f53773540b02c0ccec6
This commit is contained in:
drh
2018-03-24 18:01:51 +00:00
parent e3eff266bc
commit 821b610b63
3 changed files with 29 additions and 9 deletions
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24
src/expr.c
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@@ -5007,6 +5007,15 @@ int sqlite3ExprImpliesExpr(Parse *pParse, Expr *pE1, Expr *pE2, int iTab){
** have a non-NULL column, then set pWalker->eCode to 1 and abort.
*/
static int impliesNotNullRow(Walker *pWalker, Expr *pExpr){
/* This routine is only called for WHERE clause expressions and so it
** cannot have any TK_AGG_COLUMN entries because those are only found
** in HAVING clauses. We can get a TK_AGG_FUNCTION in a WHERE clause,
** but that is an illegal construct and the query will be rejected at
** a later stage of processing, so the TK_AGG_FUNCTION case does not
** need to be considered here. */
assert( pExpr->op!=TK_AGG_COLUMN );
testcase( pExpr->op==TK_AGG_FUNCTION );
if( ExprHasProperty(pExpr, EP_FromJoin) ) return WRC_Prune;
switch( pExpr->op ){
case TK_ISNULL:
@@ -5015,10 +5024,14 @@ static int impliesNotNullRow(Walker *pWalker, Expr *pExpr){
case TK_CASE:
case TK_IN:
case TK_FUNCTION:
case TK_AGG_FUNCTION:
testcase( pExpr->op==TK_ISNULL );
testcase( pExpr->op==TK_IS );
testcase( pExpr->op==TK_OR );
testcase( pExpr->op==TK_CASE );
testcase( pExpr->op==TK_IN );
testcase( pExpr->op==TK_FUNCTION );
return WRC_Prune;
case TK_COLUMN:
case TK_AGG_COLUMN:
if( pWalker->u.iCur==pExpr->iTable ){
pWalker->eCode = 1;
return WRC_Abort;
@@ -5035,6 +5048,13 @@ static int impliesNotNullRow(Walker *pWalker, Expr *pExpr){
** if expression p will always be NULL or false if every column of iTab
** is NULL.
**
** False negatives are acceptable. In other words, it is ok to return
** zero even if expression p will never be true of every column of iTab
** is NULL. A false negative is merely a missed optimization opportunity.
**
** False positives are not allowed, however. A false positive may result
** in an incorrect answer.
**
** Terms of p that are marked with EP_FromJoin (and hence that come from
** the ON or USING clauses of LEFT JOINS) are excluded from the analysis.
**