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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
Context
2014-05-02
14:54
Fix a broken test case in fuzz.test. check-in: faa46935 user: dan tags: trunk
13:09
Merge latest trunk enhancements and fixes into the orderby-planning branch. check-in: 84862d3a user: drh tags: orderby-planning
00:09
Add a comment explaining why WhereLoop cost adjustments are omitted for skip-scan loops. check-in: 3bc43594 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: 427409ae user: dan tags: trunk
Changes
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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;