Add a test-control to disable the skip-scan optimization.

FossilOrigin-Name: 650a3fe03d61068e06f3097878a777ca8ed713a769444cdb3f8be7d1e19a0b83

src/sqliteInt.h

@@ -1583,6 +1583,7 @@ struct sqlite3 {
    /* TH3 expects the Stat34  ^^^^^^ value to be 0x0800.  Don't change it */
 #define SQLITE_PushDown       0x1000   /* The push-down optimization */
 #define SQLITE_SimplifyJoin   0x2000   /* Convert LEFT JOIN to JOIN */
+#define SQLITE_SkipScan       0x4000   /* Skip-scans */
 #define SQLITE_AllOpts        0xffff   /* All optimizations */
 
 /*

src/test1.c

@@ -6954,6 +6954,7 @@ static int SQLITE_TCLAPI optimization_control(
     { "omit-noop-join",      SQLITE_OmitNoopJoin   },
     { "stat3",               SQLITE_Stat34         },
     { "stat4",               SQLITE_Stat34         },
+    { "skip-scan",           SQLITE_SkipScan       },
   };
 
   if( objc!=4 ){

src/where.c

@@ -2672,6 +2672,7 @@ static int whereLoopAddBtreeIndex(
   if( saved_nEq==saved_nSkip
    && saved_nEq+1<pProbe->nKeyCol
    && pProbe->noSkipScan==0
+   && OptimizationEnabled(db, SQLITE_SkipScan)
    && pProbe->aiRowLogEst[saved_nEq+1]>=42  /* TUNING: Minimum for skip-scan */
    && (rc = whereLoopResize(db, pNew, pNew->nLTerm+1))==SQLITE_OK
   ){