Do not remove the EP_CanBeNull flag from expressions during a LEFT JOIN

strength reduction if the query also contains a RIGHT JOIN. Fix for
the problem identified by
[forum/forumpost/b40696f50145d21c|forum post b40696f50145d21c].

FossilOrigin-Name: b1be2259e2e08ec22a88bc9a18b3ab4d83246ad4c635c05cdf80d3eff84df06a

manifest

@@ -1,5 +1,5 @@
-C The\ssame\srestrictions\son\sthe\suse\sof\sWHERE\sclause\sterms\sto\sdrive\sindexes\nin\sthe\spresence\sof\sRIGHT\sJOINs\salso\sapply\sto\sthe\suse\sof\sWHERE\sclause\sterms\nto\smanufacture\sautomatic\sindexes.\s\sThis\sfixes\sa\sproblem\sidentified\sby\n[forum:/forumpost/51e6959f61|forum\spost\s51e6959f61].
-D 2022-06-10T16:41:54.319
+C Do\snot\sremove\sthe\sEP_CanBeNull\sflag\sfrom\sexpressions\sduring\sa\sLEFT\sJOIN\nstrength\sreduction\sif\sthe\squery\salso\scontains\sa\sRIGHT\sJOIN.\sFix\sfor\nthe\sproblem\sidentified\sby\n[forum/forumpost/b40696f50145d21c|forum\spost\sb40696f50145d21c].
+D 2022-06-13T12:42:24.645
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -570,7 +570,7 @@ F src/printf.c 6166a30417b05c5b2f82e1f183f75faa2926ad60531c0b688a57dbc951441a20
 F src/random.c 097dc8b31b8fba5a9aca1697aeb9fd82078ec91be734c16bffda620ced7ab83c
 F src/resolve.c a4eb3c617027fd049b07432f3b942ea7151fa793a332a11a7d0f58c9539e104f
 F src/rowset.c ba9515a922af32abe1f7d39406b9d35730ed65efab9443dc5702693b60854c92
-F src/select.c 66ef4e23fea38c3306c948d0c6416862cc0ed07a01d6affba2a91001de41eec6
+F src/select.c ee3113de67330163a35307eacb4188b6778fcae1e2d2f738a9dda2daa0346e24
 F src/shell.c.in 08e59f1cb9d9b1180aba52861aaada0c95f6ddd210488719684e160a0724c806
 F src/sqlite.h.in 172528c287399a34f188154017b7268bf82c6d5b780902e361958d2318c4e37c
 F src/sqlite3.rc 5121c9e10c3964d5755191c80dd1180c122fc3a8
@@ -1167,7 +1167,7 @@ F test/join4.test 1a352e4e267114444c29266ce79e941af5885916
 F test/join5.test d22b6cba8fb59ab3f1c82701434c360705eb12d4ce200c449f37b018fc47681a
 F test/join6.test f809c025fa253f9e150c0e9afd4cef8813257bceeb6f46e04041228c9403cc2c
 F test/join7.test 2268dcbb54b724391dda3748ea95c60d960607ffeed67885675998e7117697f6
-F test/join8.test 135277faf0cab04efe42bfea9aadde034bd36dc2ce3a3de49e97c8c7f745f103
+F test/join8.test aa3294b62e6a2a7622dbab864ac924820af910309835013ffac8421c733e9f8f
 F test/join9.test 9056ddd3b0c0f4f9d658f4521038d9a37dc23ead8ca9a505d0b0db2b6a471e05
 F test/joinA.test 7eab225dc1c1ab258a5e62513a4ed7cabbd3db971d59d5d92f4fb6fa14c12f6a
 F test/joinB.test 1b2ba3fc8568b49411787fccbf540570c148e9b6a53a30f80691cb6268098ded
@@ -1976,8 +1976,8 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P e25dad868f9ef2c7847319c9f6f106999ba8b0a3e09ed9edfbd25e8fc6f3c10e
-R 71f93056490b18abe7ae590f710ffb97
+P 342c501f532523347e6c339351e02043dd6ee9e11a291224b65ea72bd6c2ba40
+R a249dd322f3d0191f7349eb16a9ed1fc
 U drh
-Z c3464bb306b9115d0a59088775264fcc
+Z 7bab454819c3e667fed9b310a0634268
 # Remove this line to create a well-formed Fossil manifest.

manifest.uuid

@@ -1 +1 @@
-342c501f532523347e6c339351e02043dd6ee9e11a291224b65ea72bd6c2ba40
+b1be2259e2e08ec22a88bc9a18b3ab4d83246ad4c635c05cdf80d3eff84df06a

src/select.c

@@ -429,15 +429,21 @@ void sqlite3SetJoinExpr(Expr *p, int iTable, u32 joinFlag){
 ** an ordinary term that omits the EP_OuterON mark.
 **
 ** This happens when a LEFT JOIN is simplified into an ordinary JOIN.
+**
+** If nullable is true, that means that Expr p might evaluate to NULL even
+** if it is a reference to a NOT NULL column.  This can happen, for example,
+** if the table that p references is on the left side of a RIGHT JOIN.
+** If nullable is true, then take care to not remove the EP_CanBeNull bit.
+** See forum thread https://sqlite.org/forum/forumpost/b40696f50145d21c
 */
-static void unsetJoinExpr(Expr *p, int iTable){
+static void unsetJoinExpr(Expr *p, int iTable, int nullable){
   while( p ){
     if( ExprHasProperty(p, EP_OuterON)
      && (iTable<0 || p->w.iJoin==iTable) ){
       ExprClearProperty(p, EP_OuterON);
       ExprSetProperty(p, EP_InnerON);
     }
-    if( p->op==TK_COLUMN && p->iTable==iTable ){
+    if( p->op==TK_COLUMN && p->iTable==iTable && !nullable ){
       ExprClearProperty(p, EP_CanBeNull);
     }
     if( p->op==TK_FUNCTION ){
@@ -445,11 +451,11 @@ static void unsetJoinExpr(Expr *p, int iTable){
       if( p->x.pList ){
         int i;
         for(i=0; i<p->x.pList->nExpr; i++){
-          unsetJoinExpr(p->x.pList->a[i].pExpr, iTable);
+          unsetJoinExpr(p->x.pList->a[i].pExpr, iTable, nullable);
         }
       }
     }
-    unsetJoinExpr(p->pLeft, iTable);
+    unsetJoinExpr(p->pLeft, iTable, nullable);
     p = p->pRight;
   } 
 }
@@ -5067,7 +5073,7 @@ static int pushDownWhereTerms(
     while( pSubq ){
       SubstContext x;
       pNew = sqlite3ExprDup(pParse->db, pWhere, 0);
-      unsetJoinExpr(pNew, -1);
+      unsetJoinExpr(pNew, -1, 1);
       x.pParse = pParse;
       x.iTable = pSrc->iCursor;
       x.iNewTable = pSrc->iCursor;
@@ -6751,7 +6757,8 @@ int sqlite3Select(
       SELECTTRACE(0x100,pParse,p,
                 ("LEFT-JOIN simplifies to JOIN on term %d\n",i));
       pItem->fg.jointype &= ~(JT_LEFT|JT_OUTER);
-      unsetJoinExpr(p->pWhere, pItem->iCursor);
+      unsetJoinExpr(p->pWhere, pItem->iCursor,
+                    pTabList->a[0].fg.jointype & JT_LTORJ);
     }
 
     /* No futher action if this term of the FROM clause is no a subquery */

test/join8.test

@@ -694,6 +694,39 @@ do_execsql_test join8-22000 {
          RIGHT JOIN t4 ON true
    WHERE a=b;
 } {}
-  
+
+# 2022-06-13
+# https://sqlite.org/forum/forumpost/b40696f501
+#
+# This optimization that converts "x ISNULL" into "FALSE" when column "x" has a
+# NOT NULL constraint is too aggresive if the query contains RIGHT JOIN.
+#
+reset_db
+db null -
+do_execsql_test join8-23000 {
+  CREATE TABLE t1(a TEXT);
+  INSERT INTO t1 VALUES('c');
+  CREATE TABLE t2(b TEXT, c TEXT NOT NULL);
+  INSERT INTO t2 VALUES('a', 'b');
+  CREATE TABLE t3(d TEXT);
+  INSERT INTO t3 VALUES('x');
+  CREATE TABLE t4(e TEXT);
+  INSERT INTO t4 VALUES('y');
+}
+do_execsql_test join8-23010 {
+  SELECT *
+    FROM t1
+         LEFT JOIN t2 ON TRUE
+         JOIN t3 ON c=''
+         RIGHT JOIN t4 ON b='';
+} {- - - - y}
+do_execsql_test join8-23020 {
+  SELECT *
+    FROM t1
+         LEFT JOIN t2 ON TRUE
+         JOIN t3 ON c=''
+         RIGHT JOIN t4 ON b=''
+   WHERE d ISNULL
+} {- - - - y}
 
 finish_test