Stricter enforcement of the idea that a MATERIALIZED common table expression

is an optimization fence.

FossilOrigin-Name: 354425f8d97437bd156265a6914f98cce91b42abc9e773453ef4d817e308dc9a

1 files changed, 15 insertions, 4 deletions

diff --git a/src/select.c b/src/select.c
index 098b74584..41e42b680 100644
--- a/src/select.c
+++ b/src/select.c
@@ -4292,7 +4292,8 @@ static int compoundHasDifferentAffinities(Select *p){
 **        (27b) the subquery is a compound query and the RIGHT JOIN occurs
 **              in any arm of the compound query.  (See also (17g).)
 **
-**  (28)  The subquery is not a MATERIALIZED CTE.
+**  (28)  The subquery is not a MATERIALIZED CTE.  (This is handled
+**        in the caller before ever reaching this routine.)
 **
 **
 ** In this routine, the "p" parameter is a pointer to the outer query.
@@ -4402,9 +4403,9 @@ static int flattenSubquery(
   if( iFrom>0 && (pSubSrc->a[0].fg.jointype & JT_LTORJ)!=0 ){
     return 0;   /* Restriction (27a) */
   }
-  if( pSubitem->fg.isCte && pSubitem->u2.pCteUse->eM10d==M10d_Yes ){
-    return 0;       /* (28) */
-  }
+
+  /* Condition (28) is blocked by the caller */
+  assert( !pSubitem->fg.isCte || pSubitem->u2.pCteUse->eM10d!=M10d_Yes );
 
   /* Restriction (17): If the sub-query is a compound SELECT, then it must
   ** use only the UNION ALL operator. And none of the simple select queries
@@ -7285,6 +7286,14 @@ 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
@@ -7314,6 +7323,8 @@ int sqlite3Select(
     **            (a)  The outer query has a different ORDER BY clause
     **            (b)  The subquery is part of a join
     **          See forum post 062d576715d277c8
+    **
+    ** Also retain the ORDER BY if the OmitOrderBy optimization is disabled.
     */
     if( pSub->pOrderBy!=0
      && (p->pOrderBy!=0 || pTabList->nSrc>1)      /* Condition (5) */