Make the value of an explicit LIMIT clause on a scalar sub-query available to xBestIndex for simple "LIMIT 0" and "LIMIT 1" queries.

FossilOrigin-Name: 33b6a63caafccc61b3193714911cd8b5dd9b7f1798054b8c7845b23688d531f2

1 files changed, 17 insertions, 11 deletions

diff --git a/src/expr.c b/src/expr.c
index c8dfd3af3..ebac654a2 100644
--- a/src/expr.c
+++ b/src/expr.c
@@ -3896,17 +3896,23 @@ int sqlite3CodeSubselect(Parse *pParse, Expr *pExpr){
     VdbeComment((v, "Init EXISTS result"));
   }
   if( pSel->pLimit ){
-    /* The subquery already has a limit.  If the pre-existing limit is X
-    ** then make the new limit X<>0 so that the new limit is either 1 or 0 */
-    sqlite3 *db = pParse->db;
-    pLimit = sqlite3Expr(db, TK_INTEGER, "0");
-    if( pLimit ){
-      pLimit->affExpr = SQLITE_AFF_NUMERIC;
-      pLimit = sqlite3PExpr(pParse, TK_NE,
-                            sqlite3ExprDup(db, pSel->pLimit->pLeft, 0), pLimit);
-    }
-    sqlite3ExprDeferredDelete(pParse, pSel->pLimit->pLeft);
-    pSel->pLimit->pLeft = pLimit;
+    /* The subquery already has a limit.  If the pre-existing limit X is 
+    ** not already integer value 1 or 0, then make the new limit X<>0 so that
+    ** the new limit is either 1 or 0 */
+    Expr *pLeft = pSel->pLimit->pLeft;
+    if( ExprHasProperty(pLeft, EP_IntValue)==0
+     || (pLeft->u.iValue!=1 && pLeft->u.iValue!=0)
+    ){
+      sqlite3 *db = pParse->db;
+      pLimit = sqlite3Expr(db, TK_INTEGER, "0");
+      if( pLimit ){
+        pLimit->affExpr = SQLITE_AFF_NUMERIC;
+        pLimit = sqlite3PExpr(pParse, TK_NE,
+            sqlite3ExprDup(db, pLeft, 0), pLimit);
+      }
+      sqlite3ExprDeferredDelete(pParse, pLeft);
+      pSel->pLimit->pLeft = pLimit;
+    }
   }else{
     /* If there is no pre-existing limit add a limit of 1 */
     pLimit = sqlite3Expr(pParse->db, TK_INTEGER, "1");