Simplify the select.c logic that enforces the optimization fence around

a MATERIALIZED common table expression.  Do not allow the
reverse_unordered_selects setting to affect a fenced CTE that itself has
an ORDER BY clause.

FossilOrigin-Name: 165b9bda243777ae7fb6f51ee7ad58771c581b82ef9a70268eb9764e9e6eba6f

2 files changed, 30 insertions, 15 deletions

diff --git a/src/select.c b/src/select.c
index 7a3ba4a6f..482d7283c 100644
--- a/src/select.c
+++ b/src/select.c
@@ -7285,6 +7285,14 @@ int sqlite3Select(
       goto select_end;
     }
 
+    /* Do not attempt the usual optimizations (flattening and ORDER BY
+    ** elimination) on a MATERIALIZED common table expression because
+    ** a MATERIALIZED common table expression is an optimization fence.
+    */
+    if( pItem->fg.isCte && pItem->u2.pCteUse->eM10d==M10d_Yes ){
+      continue;
+    }
+
     /* Do not try to flatten an aggregate subquery.
     **
     ** Flattening an aggregate subquery is only possible if the outer query
@@ -7315,21 +7323,14 @@ int sqlite3Select(
     **            (b)  The subquery is part of a join
     **          See forum post 062d576715d277c8
     **
-    ** The above cases might get an incorrect result if ORDER BY is omitted.
-    ** The following constraints are not required for correct answers, but are
-    ** included in order to give developers more control over when a sort
-    ** occurs:
-    **
-    **    (6)   The subquery is really a MATERIALIZED CTE
-    **    (7)   The OmitOrderBy optimization is disabled
+    ** Also retain the ORDER BY if the OmitOrderBy optimization is disabled.
     */
     if( pSub->pOrderBy!=0
      && (p->pOrderBy!=0 || pTabList->nSrc>1)      /* Condition (5) */
      && pSub->pLimit==0                           /* Condition (1) */
      && (pSub->selFlags & SF_OrderByReqd)==0      /* Condition (2) */
      && (p->selFlags & SF_OrderByReqd)==0         /* Condition (3) and (4) */
-     && (pItem->fg.isCte==0 || pItem->u2.pCteUse->eM10d!=M10d_Yes) /* (6) */
-     && OptimizationEnabled(db, SQLITE_OmitOrderBy)                /* (7) */
+     && OptimizationEnabled(db, SQLITE_OmitOrderBy)
     ){
       TREETRACE(0x800,pParse,p,
                 ("omit superfluous ORDER BY on %r FROM-clause subquery\n",i+1));
@@ -7366,11 +7367,6 @@ int sqlite3Select(
       continue;
     }
 
-    /* Do not flatten a MATERIALIZED CTE */
-    if( pItem->fg.isCte && pItem->u2.pCteUse->eM10d==M10d_Yes ){
-      continue;
-    }
-
     if( flattenSubquery(pParse, p, i, isAgg) ){
       if( pParse->nErr ) goto select_end;
       /* This subquery can be absorbed into its parent. */
diff --git a/src/where.c b/src/where.c
index c75834ff5..9992b2053 100644
--- a/src/where.c
+++ b/src/where.c
@@ -6122,8 +6122,27 @@ WhereInfo *sqlite3WhereBegin(
        if( db->mallocFailed ) goto whereBeginError;
     }
   }
+  assert( pWInfo->pTabList!=0 );
   if( pWInfo->pOrderBy==0 && (db->flags & SQLITE_ReverseOrder)!=0 ){
-     pWInfo->revMask = ALLBITS;
+    for(ii=0; ii<pWInfo->pTabList->nSrc; ii++){
+      /* The PRAGMA reverse_unordered_selects=ON setting (also accessible
+      ** using SQLITE_DBCONFIG_REVERSE_SCANORDER) means to reverse the scan
+      ** order for any table that is part of a query that does not have an
+      ** ORDER BY clause.
+      **
+      ** Except, do not reverse the output from MATERIALIZED common table
+      ** expression that has an internal ORDER BY clause, because a
+      ** MATERIALIZED common table expression is an optimization fence.
+      */
+      SrcItem *pItem = &pWInfo->pTabList->a[ii];
+      if( !pItem->fg.isCte
+       || pItem->u2.pCteUse->eM10d!=M10d_Yes
+       || NEVER(pItem->pSelect==0)
+       || pItem->pSelect->pOrderBy==0
+      ){
+        pWInfo->revMask |= MASKBIT(ii);
+      }
+    }
   }
   if( pParse->nErr ){
     goto whereBeginError;