The NOT NULL strength reduction optimization from [de9c86c9e4cdb34f] should

be applied to the WHERE clause only.  Otherwise, the operand of the IS NULL
or IS NOT NULL operator might be a reference to a bare column of an
aggregate table, and we can't tell if it is NULL or not based only on its
NOT NULL attribute.  [forum:/forumpost/440f2a2f17|Forum post 440f2a2f17].

FossilOrigin-Name: 51704feae224eff601db5607f8651da11b3c2ed8a58ffe5b6ee8260cab50695b

2 files changed, 44 insertions, 10 deletions

diff --git a/src/resolve.c b/src/resolve.c
index 5d0801e82..c2957a870 100644
--- a/src/resolve.c
+++ b/src/resolve.c
@@ -971,6 +971,19 @@ static int resolveExprStep(Walker *pWalker, Expr *pExpr){
     ** resolved.  This prevents "column" from being counted as having been
     ** referenced, which might prevent a SELECT from being erroneously
     ** marked as correlated.
+    **
+    ** 2024-03-28: Beware of aggregates.  A bare column of aggregated table
+    ** can still evaluate to NULL even though it is marked as NOT NULL.
+    ** Example:
+    **
+    **       CREATE TABLE t1(a INT NOT NULL);
+    **       SELECT a, a IS NULL, a IS NOT NULL, count(*) FROM t1;
+    **
+    ** The "a IS NULL" and "a IS NOT NULL" expressions cannot be optimized
+    ** here because at the time this case is hit, we do not yet know whether
+    ** or not t1 is being aggregated.  We have to assume the worst and omit
+    ** the optimization.  The only time it is safe to apply this optimization
+    ** is within the WHERE clause.
     */
     case TK_NOTNULL:
     case TK_ISNULL: {
@@ -981,19 +994,36 @@ static int resolveExprStep(Walker *pWalker, Expr *pExpr){
         anRef[i] = p->nRef;
       }
       sqlite3WalkExpr(pWalker, pExpr->pLeft);
-      if( 0==sqlite3ExprCanBeNull(pExpr->pLeft) && !IN_RENAME_OBJECT ){
-        testcase( ExprHasProperty(pExpr, EP_OuterON) );
-        assert( !ExprHasProperty(pExpr, EP_IntValue) );
-        pExpr->u.iValue = (pExpr->op==TK_NOTNULL);
-        pExpr->flags |= EP_IntValue;
-        pExpr->op = TK_INTEGER;
+      if( IN_RENAME_OBJECT ) return WRC_Prune;
+      if( sqlite3ExprCanBeNull(pExpr->pLeft) ){
+        /* The expression can be NULL.  So the optimization does not apply */
+        return WRC_Prune;
+      }
 
-        for(i=0, p=pNC; p && i<ArraySize(anRef); p=p->pNext, i++){
-          p->nRef = anRef[i];
+      for(i=0, p=pNC; p; p=p->pNext, i++){
+        if( (p->ncFlags & NC_Where)==0 ){
+          return WRC_Prune;  /* Not in a WHERE clause.  Unsafe to optimize. */
         }
-        sqlite3ExprDelete(pParse->db, pExpr->pLeft);
-        pExpr->pLeft = 0;
       }
+      testcase( ExprHasProperty(pExpr, EP_OuterON) );
+      assert( !ExprHasProperty(pExpr, EP_IntValue) );
+#if TREETRACE_ENABLED
+      if( sqlite3TreeTrace & 0x80000 ){
+        sqlite3DebugPrintf(
+           "NOT NULL strength reduction converts the following to %d:\n",
+           pExpr->op==TK_NOTNULL
+        );
+        sqlite3ShowExpr(pExpr);
+      }
+#endif /* TREETRACE_ENABLED */
+      pExpr->u.iValue = (pExpr->op==TK_NOTNULL);
+      pExpr->flags |= EP_IntValue;
+      pExpr->op = TK_INTEGER;
+      for(i=0, p=pNC; p && i<ArraySize(anRef); p=p->pNext, i++){
+        p->nRef = anRef[i];
+      }
+      sqlite3ExprDelete(pParse->db, pExpr->pLeft);
+      pExpr->pLeft = 0;
       return WRC_Prune;
     }
 
@@ -1891,7 +1921,9 @@ static int resolveSelectStep(Walker *pWalker, Select *p){
       }
       if( sqlite3ResolveExprNames(&sNC, p->pHaving) ) return WRC_Abort;
     }
+    sNC.ncFlags |= NC_Where;
     if( sqlite3ResolveExprNames(&sNC, p->pWhere) ) return WRC_Abort;
+    sNC.ncFlags &= ~NC_Where;
 
     /* Resolve names in table-valued-function arguments */
     for(i=0; i<p->pSrc->nSrc; i++){
diff --git a/src/sqliteInt.h b/src/sqliteInt.h
index 91cf17304..e11b5b3a4 100644
--- a/src/sqliteInt.h
+++ b/src/sqliteInt.h
@@ -1125,6 +1125,7 @@ extern u32 sqlite3TreeTrace;
 **   0x00010000     Beginning of DELETE/INSERT/UPDATE processing
 **   0x00020000     Transform DISTINCT into GROUP BY
 **   0x00040000     SELECT tree dump after all code has been generated
+**   0x00080000     NOT NULL strength reduction
 */
 
 /*
@@ -3453,6 +3454,7 @@ struct NameContext {
 #define NC_InAggFunc 0x020000 /* True if analyzing arguments to an agg func */
 #define NC_FromDDL   0x040000 /* SQL text comes from sqlite_schema */
 #define NC_NoSelect  0x080000 /* Do not descend into sub-selects */
+#define NC_Where     0x100000 /* Processing WHERE clause of a SELECT */
 #define NC_OrderAgg 0x8000000 /* Has an aggregate other than count/min/max */
 
 /*