Index: src/where.c
==================================================================
--- src/where.c
+++ src/where.c
@@ -4289,24 +4289,44 @@
 
 /*
 ** Adjust the WhereLoop.nOut value downward to account for terms of the
 ** WHERE clause that reference the loop but which are not used by an
 ** index.
+*
+** For every WHERE clause term that is not used by the index
+** and which has a truth probability assigned by one of the likelihood(),
+** likely(), or unlikely() SQL functions, reduce the estimated number
+** of output rows by the probability specified.
 **
-** In the current implementation, the first extra WHERE clause term reduces
-** the number of output rows by a factor of 10 and each additional term
-** reduces the number of output rows by sqrt(2).
+** TUNING:  For every WHERE clause term that is not used by the index
+** and which does not have an assigned truth probability, heuristics
+** described below are used to try to estimate the truth probability.
+** TODO --> Perhaps this is something that could be improved by better
+** table statistics.
+**
+** Heuristic 1:  Estimate the truth probability as 6.25%.  The 6.25%
+** value corresponds to 1 in LogEst notation, so this means decrement
+** the WhereLoop.nOut field for every such WHERE clause term.
+**
+** Heuristic 2:  If there exists one or more WHERE clause terms of the
+** form "x==EXPR" and EXPR is not a constant 0 or 1, then make sure the
+** final output row estimate is no greater than 1/4 of the total number
+** of rows in the table.  In other words, assume that x==EXPR will filter
+** out at least 3 out of 4 rows.  If EXPR is -1 or 0 or 1, then maybe the
+** "x" column is boolean or else -1 or 0 or 1 is a common default value
+** on the "x" column and so in that case only cap the output row estimate
+** at 1/2 instead of 1/4.
 */
 static void whereLoopOutputAdjust(
   WhereClause *pWC,      /* The WHERE clause */
   WhereLoop *pLoop,      /* The loop to adjust downward */
   LogEst nRow            /* Number of rows in the entire table */
 ){
   WhereTerm *pTerm, *pX;
   Bitmask notAllowed = ~(pLoop->prereq|pLoop->maskSelf);
-  int i, j;
-  int nEq = 0;    /* Number of = constraints not within likely()/unlikely() */
+  int i, j, k;
+  LogEst iReduce = 0;    /* pLoop->nOut should not exceed nRow-iReduce */
 
   assert( (pLoop->wsFlags & WHERE_AUTO_INDEX)==0 );
   for(i=pWC->nTerm, pTerm=pWC->a; i>0; i--, pTerm++){
     if( (pTerm->wtFlags & TERM_VIRTUAL)!=0 ) break;
     if( (pTerm->prereqAll & pLoop->maskSelf)==0 ) continue;
@@ -4317,24 +4337,30 @@
       if( pX==pTerm ) break;
       if( pX->iParent>=0 && (&pWC->a[pX->iParent])==pTerm ) break;
     }
     if( j<0 ){
       if( pTerm->truthProb<=0 ){
+        /* If a truth probability is specified using the likelihood() hints,
+        ** then use the probability provided by the application. */
         pLoop->nOut += pTerm->truthProb;
       }else{
+        /* In the absence of explicit truth probabilities, use heuristics to
+        ** guess a reasonable truth probability. */
         pLoop->nOut--;
-        if( pTerm->eOperator&WO_EQ ) nEq++;
+        if( pTerm->eOperator&WO_EQ ){
+          Expr *pRight = pTerm->pExpr->pRight;
+          if( sqlite3ExprIsInteger(pRight, &k) && k>=(-1) && k<=1 ){
+            k = 10;
+          }else{
+            k = 20;
+          }
+          if( iReduce<k ) iReduce = k;
+        }
       }
     }
   }
-  /* TUNING:  If there is at least one equality constraint in the WHERE
-  ** clause that does not have a likelihood() explicitly assigned to it
-  ** then do not let the estimated number of output rows exceed half 
-  ** the number of rows in the table. */
-  if( nEq && pLoop->nOut>nRow-10 ){
-    pLoop->nOut = nRow - 10;
-  }
+  if( pLoop->nOut > nRow-iReduce )  pLoop->nOut = nRow - iReduce;
 }
 
 /*
 ** Adjust the cost C by the costMult facter T.  This only occurs if
 ** compiled with -DSQLITE_ENABLE_COSTMULT

Index: test/scanstatus.test
==================================================================
--- test/scanstatus.test
+++ test/scanstatus.test
@@ -266,11 +266,11 @@
 do_execsql_test    4.2.1 { DELETE FROM p1 WHERE x=4 }
 do_scanstatus_test 4.2.2 { 
   nLoop 1 nVisit 1 nEst 1.0 zName sqlite_autoindex_p1_1 
   zExplain {SEARCH TABLE p1 USING INDEX sqlite_autoindex_p1_1 (x=?)}
 
-  nLoop 1 nVisit 3 nEst 524288.0 zName c1 zExplain {SCAN TABLE c1}
+  nLoop 1 nVisit 3 nEst 262144.0 zName c1 zExplain {SCAN TABLE c1}
 }
 
 #-------------------------------------------------------------------------
 # Further tests of different scan types.
 #