Improvements to the query planner to address the inefficiency described

by [forum/forumpost/2568d1f6e6|forum post 2568d1f6e6].

FossilOrigin-Name: 72fcc12cda910a0e3f7875eb3d117b2a5608705c97703985427a02960f1ab5c5

1 files changed, 37 insertions, 23 deletions

diff --git a/src/where.c b/src/where.c
index cae3dd074..0b56de6c1 100644
--- a/src/where.c
+++ b/src/where.c
@@ -2430,46 +2430,60 @@ static void whereInfoFree(sqlite3 *db, WhereInfo *pWInfo){
 }
 
 /*
-** Return TRUE if all of the following are true:
-**
-**   (1)  X has the same or lower cost, or returns the same or fewer rows,
-**        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.  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.
+** Return TRUE if X is a proper subset of Y but is of equal or less cost.
+** In other words, return true if all constraints of X are also part of Y
+** and Y has additional constraints that might speed the search that X lacks
+** but the cost of running X is not more than the cost of running Y.
+**
+** In other words, return true if the cost relationwship between X and Y
+** is inverted and needs to be adjusted.
+**
+** Case 1:
+**
+**   (1a)  X and Y use the same index.
+**   (1b)  X has fewer == terms than Y
+**   (1c)  Neither X nor Y use skip-scan
+**   (1d)  X does not have a a greater cost than Y
+**
+** Case 2:
+**
+**   (2a)  X has the same or lower cost, or returns the same or fewer rows,
+**         than Y.
+**   (2b)  X uses fewer WHERE clause terms than Y
+**   (2c)  Every WHERE clause term used by X is also used by Y
+**   (2d)  X skips at least as many columns as Y
+**   (2e)  If X is a covering index, than Y is too
 */
 static int whereLoopCheaperProperSubset(
   const WhereLoop *pX,       /* First WhereLoop to compare */
   const WhereLoop *pY        /* Compare against this WhereLoop */
 ){
   int i, j;
+  if( pX->rRun>pY->rRun && pX->nOut>pY->nOut ) return 0; /* (1d) and (2a) */
+  assert( (pX->wsFlags & WHERE_VIRTUALTABLE)==0 );
+  assert( (pY->wsFlags & WHERE_VIRTUALTABLE)==0 );
+  if( pX->u.btree.nEq < pY->u.btree.nEq                  /* (1b) */
+   && pX->u.btree.pIndex==pY->u.btree.pIndex             /* (1a) */
+   && pX->nSkip==0 && pY->nSkip==0                       /* (1c) */
+  ){
+    return 1;  /* Case 1 is true */
+  }
   if( pX->nLTerm-pX->nSkip >= pY->nLTerm-pY->nSkip ){
-    return 0; /* X is not a subset of Y */
+    return 0;                                            /* (2b) */
   }
-  if( pX->rRun>pY->rRun && pX->nOut>pY->nOut ) return 0;
-  if( pY->nSkip > pX->nSkip ) return 0;
+  if( pY->nSkip > pX->nSkip ) return 0;                  /* (2d) */
   for(i=pX->nLTerm-1; i>=0; i--){
     if( pX->aLTerm[i]==0 ) continue;
     for(j=pY->nLTerm-1; j>=0; j--){
       if( pY->aLTerm[j]==pX->aLTerm[i] ) break;
     }
-    if( j<0 ) return 0;  /* X not a subset of Y since term X[i] not used by Y */
+    if( j<0 ) return 0;                                  /* (2c) */
   }
   if( (pX->wsFlags&WHERE_IDX_ONLY)!=0
    && (pY->wsFlags&WHERE_IDX_ONLY)==0 ){
-    return 0;  /* Constraint (5) */
+    return 0;                                            /* (2e) */
   }
-  return 1;  /* All conditions meet */
+  return 1;  /* Case 2 is true */
 }
 
 /*