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Overview
Comment:Improved comments, including a typo fix, on the whereLoopCheaperProperSubset() function.
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SHA3-256: 700a3c694438ca6cca185d0097f24799e82717ef38cb47bd83666c80f0e3cb2f
User & Date: drh 2017-10-16 11:50:12.951
Context
2017-10-17
13:44
Fix the ".check" command of the command-line shell to return 1 (error) rather than 2 (exit) on a test failure. (check-in: e2af0cc6ef user: drh tags: trunk)
2017-10-16
11:50
Improved comments, including a typo fix, on the whereLoopCheaperProperSubset() function. (check-in: 700a3c6944 user: drh tags: trunk)
2017-10-15
22:16
In the query planner, do not consider index X to be a proper subset of index Y if X is a covering index but Y is not. (check-in: ee31c04353 user: drh tags: trunk)
Changes
Side-by-Side Diff Ignore Whitespace Patch
Changes to src/where.c.
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  }
}

/*
** Return TRUE if all of the following are true:
**
**   (1)  X has the same or lower cost that Y
**   (2)  X users fewer WHERE clause terms than Y
**   (2)  X uses fewer WHERE clause terms than Y
**   (3)  Every WHERE clause term used by X is also used by Y
**   (4)  X skips at least as many columns as Y
**   (5)  If X is a covering index, than Y is too
**
** Conditions (2) and (3) mean that X is a "proper subset" of Y.
** If X is a proper subset of Y then Y is a better choice and ought
** to have a lower cost.  This routine returns TRUE when that cost 
** relationship is inverted and needs to be adjusted.  Constraint (4)
** was added because if X uses skip-scan less than Y it still might
** deserve a lower cost even if it is a proper subset of Y.
** deserve a lower cost even if it is a proper subset of Y.  Constraint (5)
** was added because a covering index probably deserves to have a lower cost
** than a non-covering index even if it is a proper subset.
*/
static int whereLoopCheaperProperSubset(
  const WhereLoop *pX,       /* First WhereLoop to compare */
  const WhereLoop *pY        /* Compare against this WhereLoop */
){
  int i, j;
  if( pX->nLTerm-pX->nSkip >= pY->nLTerm-pY->nSkip ){
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