Remove a condition from query flattening that is now taken care of by

the caller.  Factor out the reverse_unordered_selects processing from the
main loop of sqlite3WhereBegin() for performance.

FossilOrigin-Name: f068f105fb158634321bf6401f0774c81059932d213a18b627ae98bcffc10912

2 files changed, 28 insertions, 23 deletions

diff --git a/src/select.c b/src/select.c
index 482d7283c..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
diff --git a/src/where.c b/src/where.c
index 9992b2053..213df4223 100644
--- a/src/where.c
+++ b/src/where.c
@@ -5759,6 +5759,28 @@ static SQLITE_NOINLINE void whereAddIndexedExpr(
 }
 
 /*
+** Set the reverse-scan order mask to one for all tables in the query
+** with the exception of MATERIALIZED common table expressions that have
+** their own internal ORDER BY clauses.
+**
+** This implements the PRAGMA reverse_unordered_selects=ON setting.
+** (Also SQLITE_DBCONFIG_REVERSE_SCANORDER).
+*/
+static SQLITE_NOINLINE void whereReverseScanOrder(WhereInfo *pWInfo){
+  int ii;
+  for(ii=0; ii<pWInfo->pTabList->nSrc; ii++){
+    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);
+    }
+  }
+}
+
+/*
 ** Generate the beginning of the loop used for WHERE clause processing.
 ** The return value is a pointer to an opaque structure that contains
 ** information needed to terminate the loop.  Later, the calling routine
@@ -6124,25 +6146,7 @@ WhereInfo *sqlite3WhereBegin(
   }
   assert( pWInfo->pTabList!=0 );
   if( pWInfo->pOrderBy==0 && (db->flags & SQLITE_ReverseOrder)!=0 ){
-    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);
-      }
-    }
+    whereReverseScanOrder(pWInfo);
   }
   if( pParse->nErr ){
     goto whereBeginError;