Index: src/resolve.c
==================================================================
--- src/resolve.c
+++ src/resolve.c
@@ -782,10 +782,48 @@
       pExpr->iTable = pItem->iCursor;
       pExpr->iColumn--;
       pExpr->affExpr = SQLITE_AFF_INTEGER;
       break;
     }
+
+    /* An "<expr> IS NOT NULL" or "<expr> IS NULL". After resolving the
+    ** LHS, check if there is a NOT NULL constraint in the schema that
+    ** means the value of the expression can be determined immediately.
+    ** If that is the case, replace the current expression node with 
+    ** a TK_TRUEFALSE node.
+    **
+    ** If the node is replaced with a TK_TRUEFALSE node, then also restore
+    ** the NameContext ref-counts to the state they where in before the
+    ** LHS expression was resolved. This prevents the current select
+    ** from being erroneously marked as correlated in some cases.
+    */
+    case TK_NOTNULL:
+    case TK_ISNULL: {
+      int anRef[8];
+      NameContext *p;
+      int i;
+      for(i=0, p=pNC; p && i<ArraySize(anRef); p=p->pNext, i++){ 
+        anRef[i] = p->nRef;
+      }
+      sqlite3WalkExpr(pWalker, pExpr->pLeft);
+      if( 0==sqlite3ExprCanBeNull(pExpr->pLeft) ){
+        if( pExpr->op==TK_NOTNULL ){
+          pExpr->u.zToken = "true";
+          ExprSetProperty(pExpr, EP_IsTrue);
+        }else{
+          pExpr->u.zToken = "false";
+          ExprSetProperty(pExpr, EP_IsFalse);
+        }
+        pExpr->op = TK_TRUEFALSE;
+        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;
+    }
 
     /* A column name:                    ID
     ** Or table name and column name:    ID.ID
     ** Or a database, table and column:  ID.ID.ID
     **

Index: src/select.c
==================================================================
--- src/select.c
+++ src/select.c
@@ -404,10 +404,13 @@
 static void unsetJoinExpr(Expr *p, int iTable){
   while( p ){
     if( ExprHasProperty(p, EP_FromJoin)
      && (iTable<0 || p->iRightJoinTable==iTable) ){
       ExprClearProperty(p, EP_FromJoin);
+    }
+    if( p->op==TK_COLUMN && p->iTable==iTable ){
+      ExprClearProperty(p, EP_CanBeNull);
     }
     if( p->op==TK_FUNCTION && p->x.pList ){
       int i;
       for(i=0; i<p->x.pList->nExpr; i++){
         unsetJoinExpr(p->x.pList->a[i].pExpr, iTable);

Index: src/whereexpr.c
==================================================================
--- src/whereexpr.c
+++ src/whereexpr.c
@@ -1405,10 +1405,16 @@
       pNew->u.x.leftColumn = aiCurCol[1];
       testcase( (prereqLeft | extraRight) != prereqLeft );
       pNew->prereqRight = prereqLeft | extraRight;
       pNew->prereqAll = prereqAll;
       pNew->eOperator = (operatorMask(pDup->op) + eExtraOp) & opMask;
+    }else if( op==TK_ISNULL && 0==sqlite3ExprCanBeNull(pLeft) ){
+      pExpr->op = TK_TRUEFALSE;
+      pExpr->u.zToken = "false";
+      ExprSetProperty(pExpr, EP_IsFalse);
+      pTerm->prereqAll = 0;
+      pTerm->eOperator = 0;
     }
   }
 
 #ifndef SQLITE_OMIT_BETWEEN_OPTIMIZATION
   /* If a term is the BETWEEN operator, create two new virtual terms

ADDED   test/notnull2.test
Index: test/notnull2.test
==================================================================
--- /dev/null
+++ test/notnull2.test
@@ -0,0 +1,101 @@
+# 2021 February 15
+#
+# The author disclaims copyright to this source code.  In place of
+# a legal notice, here is a blessing:
+#
+#    May you do good and not evil.
+#    May you find forgiveness for yourself and forgive others.
+#    May you share freely, never taking more than you give.
+#
+#***********************************************************************
+# This file implements regression tests for SQLite library.  The
+# focus of this file is testing optimizations associated with "IS NULL"
+# and "IS NOT NULL" operators on columns with NOT NULL constraints.
+#
+
+set testdir [file dirname $argv0]
+source $testdir/tester.tcl
+set testprefix notnull2
+
+do_execsql_test 1.0 {
+  CREATE TABLE t1(a, b);
+  CREATE TABLE t2(c, d NOT NULL);
+
+  WITH x(i) AS (
+    SELECT 1 UNION ALL SELECT i+1 FROM x WHERE i<1000
+  )
+  INSERT INTO t1 SELECT i, i FROM x;
+  INSERT INTO t2 SELECT * FROM t1;
+}
+
+proc do_vmstep_test {tn sql nstep {res {}}} {
+  uplevel [list do_execsql_test $tn.0 $sql $res]
+
+  set vmstep [db status vmstep]
+  if {[string range $nstep 0 0]=="+"} {
+    set body "if {$vmstep<$nstep} {
+      error \"got $vmstep, expected more than [string range $nstep 1 end]\"
+    }"
+  } else {
+    set body "if {$vmstep>$nstep} {
+      error \"got $vmstep, expected less than $nstep\"
+    }"
+  }
+
+  # set name "$tn.vmstep=$vmstep,expect=$nstep"
+  set name "$tn.1"
+  uplevel [list do_test $name $body {}]
+}
+
+do_vmstep_test 1.1.1 {
+  SELECT * FROM t1 LEFT JOIN t2 WHERE a=c AND d IS NULL;
+} 100 {}
+do_vmstep_test 1.1.2 {
+  SELECT * FROM t1 LEFT JOIN t2 WHERE a=c AND c IS NULL;
+} +1000 {}
+
+do_vmstep_test 1.2.1 {
+  SELECT * FROM ( SELECT * FROM t2 ) WHERE d IS NULL
+} 100 {}
+do_vmstep_test 1.2.2 {
+  SELECT * FROM ( SELECT * FROM t2 ) WHERE c IS NULL
+} +1000 {}
+
+do_vmstep_test 1.3.1 {
+  SELECT * FROM t2 WHERE d IS NULL
+} 100 {}
+do_vmstep_test 1.3.2 {
+  SELECT * FROM t2 WHERE c IS NULL
+} +1000 {}
+
+do_vmstep_test 1.4.1 {
+  SELECT (d IS NOT NULL) FROM t2 WHERE 0==( d IS NOT NULL )
+} 100 {}
+do_vmstep_test 1.4.2 {
+  SELECT * FROM t2 WHERE 0==( c IS NOT NULL )
+} +1000 {}
+
+do_vmstep_test 1.5.1 {
+  SELECT count(*) FROM t2 WHERE EXISTS(
+    SELECT t2.d IS NULL FROM t1 WHERE t1.a=450
+  )
+} 10000 {1000}
+do_vmstep_test 1.5.2 {
+  SELECT count(*) FROM t2 WHERE EXISTS(
+    SELECT t2.c IS NULL FROM t1 WHERE t1.a=450
+  )
+} +100000 {1000}
+
+#-------------------------------------------------------------------------
+reset_db
+do_execsql_test 2.0 {
+  CREATE TABLE T1(a INTEGER PRIMARY KEY, b);
+  CREATE TABLE T3(k, v);
+}
+
+do_execsql_test 2.1 {
+  SELECT * FROM (SELECT a, b FROM t1) LEFT JOIN t3 ON a IS NULL;
+}
+
+finish_test
+