Make handling of LIMIT clauses in correlated sub-queries on virtual tables more efficient.

FossilOrigin-Name: 7214cb2a5b35190a06a1040cd4c54f325d347f8d8e36a07fd76c0a421e266522

1 files changed, 17 insertions, 3 deletions

diff --git a/src/whereexpr.c b/src/whereexpr.c
index 53c8508e5..fc7a30a42 100644
--- a/src/whereexpr.c
+++ b/src/whereexpr.c
@@ -1652,7 +1652,7 @@ static void whereAddLimitExpr(
 **
 **   1. The SELECT statement has a LIMIT clause, and
 **   2. The SELECT statement is not an aggregate or DISTINCT query, and
-**   3. The SELECT statement has exactly one object in its from clause, and
+**   3. The SELECT statement has exactly one object in its FROM clause, and
 **      that object is a virtual table, and
 **   4. There are no terms in the WHERE clause that will not be passed
 **      to the virtual table xBestIndex method.
@@ -1689,8 +1689,22 @@ void SQLITE_NOINLINE sqlite3WhereAddLimit(WhereClause *pWC, Select *p){
         ** (leftCursor==iCsr) test below.  */
         continue;
       }
-      if( pWC->a[ii].leftCursor!=iCsr ) return;
-      if( pWC->a[ii].prereqRight!=0 ) return;
+      if( pWC->a[ii].leftCursor==iCsr && pWC->a[ii].prereqRight==0 ) continue;
+
+      /* If this term has a parent with exactly one child, and the parent will
+      ** be passed through to xBestIndex, then this term can be ignored.  */
+      if( pWC->a[ii].iParent>=0 ){
+        WhereTerm *pParent = &pWC->a[ pWC->a[ii].iParent ];
+        if( pParent->leftCursor==iCsr
+         && pParent->prereqRight==0
+         && ALWAYS(pParent->nChild==1)
+        ){
+          continue;
+        }
+      }
+
+      /* This term will not be passed through. Do not add a LIMIT clause. */
+      return;
     }
 
     /* Check condition (5). Return early if it is not met. */