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Improved comments on the new query flattener restriction of the previous

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check-in.  Also a NEVER() macro on an unreachable branch.

FossilOrigin-Name: 8c9e2d6315fde014bd6e25a7fa58ba054f5328c8a56e9d3d1c2d069c025a1b03
This commit is contained in:
drh
2022-06-08 17:48:14 +00:00
parent 40357f0d5d
commit 8a5469b6f5
3 changed files with 26 additions and 11 deletions
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23
src/select.c
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@@ -4302,14 +4302,29 @@ static int flattenSubquery(
return 0; /* (28) */
}
/* Restriction (29):
**
** We do not want two constraints on the same term of the flattened
** query where one constraint has EP_InnerON and the other is EP_OuterON.
** To prevent this, one or the other of the following conditions must be
** false:
**
** (29a) The right-most entry in the FROM clause of the subquery
** must not be part of an outer join.
**
** (29b) The subquery itself must not be the right operand of a
** NATURAL join or a join that as an ON or USING clause.
**
** These conditions are sufficient to keep an EP_OuterON from being
** flattened into an EP_InnerON. Restrictions (3a) and (27) prevent
** an EP_InnerON from being flattened into an EP_OuterON.
*/
if( pSubSrc->nSrc>=2
&& (pSubSrc->a[pSubSrc->nSrc-1].fg.jointype & JT_OUTER)!=0
){
/* Reach here if the right-most term in the FROM clause of the subquery
** is an outer join - the second half of (29) */
if( pSubitem->fg.jointype & JT_NATURAL
if( (pSubitem->fg.jointype & JT_NATURAL)!=0
|| pSubitem->fg.isUsing
|| pSubitem->u3.pOn!=0
|| NEVER(pSubitem->u3.pOn!=0) /* ON clause already shifted into WHERE */
|| pSubitem->fg.isOn
){
return 0;