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Overview
Comment:Add a comment explaining why WhereLoop cost adjustments are omitted for skip-scan loops.
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SHA1: 3bc43594aaeee9225c0590677fcce480bedcb37b
User & Date: drh 2014-05-02 00:09:40.134
Context
2014-05-02
14:54
Fix a broken test case in fuzz.test. (check-in: faa469355e user: dan tags: trunk)
13:09
Merge latest trunk enhancements and fixes into the orderby-planning branch. (check-in: 84862d3a09 user: drh tags: orderby-planning)
00:09
Add a comment explaining why WhereLoop cost adjustments are omitted for skip-scan loops. (check-in: 3bc43594aa user: drh tags: trunk)
2014-05-01
20:26
Fix an obscure problem to do with temp register allocation that could occur if more than one simple SELECT within a compound SELECT uses a partial sort. (check-in: 427409ae10 user: dan tags: trunk)
Changes
Unified Diff Show Whitespace Changes Patch
Changes to src/where.c.
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**
**   (2) pTemplate costs more than any other WhereLoops for which pTemplate
**       is a proper subset.
**
** To say "WhereLoop X is a proper subset of Y" means that X uses fewer
** WHERE clause terms than Y and that every WHERE clause term used by X is
** also used by Y.











*/
static void whereLoopAdjustCost(const WhereLoop *p, WhereLoop *pTemplate){
  if( (pTemplate->wsFlags & WHERE_INDEXED)==0 ) return;
  if( (pTemplate->wsFlags & WHERE_SKIPSCAN)!=0 ) return;
  for(; p; p=p->pNextLoop){
    if( p->iTab!=pTemplate->iTab ) continue;
    if( (p->wsFlags & WHERE_INDEXED)==0 ) continue;







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**
**   (2) pTemplate costs more than any other WhereLoops for which pTemplate
**       is a proper subset.
**
** To say "WhereLoop X is a proper subset of Y" means that X uses fewer
** WHERE clause terms than Y and that every WHERE clause term used by X is
** also used by Y.
**
** This adjustment is omitted for SKIPSCAN loops.  In a SKIPSCAN loop, the
** WhereLoop.nLTerm field is not an accurate measure of the number of WHERE
** clause terms covered, since some of the first nLTerm entries in aLTerm[]
** will be NULL (because they are skipped).  That makes it more difficult
** to compare the loops.  We could add extra code to do the comparison, and
** perhaps we will someday.  But SKIPSCAN is sufficiently uncommon, and this
** adjustment is sufficient minor, that it is very difficult to construct
** a test case where the extra code would improve the query plan.  Better
** to avoid the added complexity and just omit cost adjustments to SKIPSCAN
** loops.
*/
static void whereLoopAdjustCost(const WhereLoop *p, WhereLoop *pTemplate){
  if( (pTemplate->wsFlags & WHERE_INDEXED)==0 ) return;
  if( (pTemplate->wsFlags & WHERE_SKIPSCAN)!=0 ) return;
  for(; p; p=p->pNextLoop){
    if( p->iTab!=pTemplate->iTab ) continue;
    if( (p->wsFlags & WHERE_INDEXED)==0 ) continue;