1 files changed, 36 insertions, 0 deletions
diff --git a/src/expr.c b/src/expr.c
index 170987621..addd5794d 100644
--- a/src/expr.c
+++ b/src/expr.c
@@ -2296,6 +2296,42 @@ int sqlite3ExprIsTableConstant(Expr *p, int iCur){
   return exprIsConst(p, 3, iCur);
 }
 
+/*
+** Check pExpr to see if it is an invariant constraint on data source pSrc.
+** This is an optimization.  False negatives will perhaps cause slower
+** queries, but false positives will yield incorrect answers.  So when in
+** double, return 0.
+**
+** To be an invariant constraint, the following must be true:
+**
+**   (1)  pExpr cannot refer to any table other than pSrc->iCursor.
+**
+**   (2)  pExpr cannot use subqueries or non-deterministic functions.
+**
+**   (3)  pSrc cannot be part of the left operand for a RIGHT JOIN.
+**        (Is there some way to relax this constraint?)
+**
+**   (4)  If pSrc is the right operand of a LEFT JOIN, then...
+**         (4a)  pExpr must come from an ON clause..
+           (4b)  and specifically the ON clause associated with the LEFT JOIN.
+**
+**   (5)  If pSrc is not the right operand of a LEFT JOIN or the left
+**        operand of a RIGHT JOIN, then pExpr must be from the WHERE
+**        clause, not an ON clause.
+*/
+int sqlite3ExprIsTableConstraint(Expr *pExpr, const SrcItem *pSrc){
+  if( pSrc->fg.jointype & JT_LTORJ ){
+    return 0;  /* rule (3) */
+  }
+  if( pSrc->fg.jointype & JT_LEFT ){
+    if( !ExprHasProperty(pExpr, EP_FromJoin) ) return 0;  /* rule (4a) */
+    if( pExpr->w.iJoin!=pSrc->iCursor ) return 0;         /* rule (4b) */
+  }else{
+    if( ExprHasProperty(pExpr, EP_FromJoin) ) return 0;   /* rule (5) */
+  }
+  return sqlite3ExprIsTableConstant(pExpr, pSrc->iCursor); /* rules (1), (2) */
+}
+
 
 /*
 ** sqlite3WalkExpr() callback used by sqlite3ExprIsConstantOrGroupBy().