An initial attempt to optimize VIEWs that occur as the right operand of a

LEFT JOIN.  This particular check-in does not work correctly because it does
not deal with the case of columns in the VIEW that return non-NULL even when
all columns in the table of the VIEW are NULL because of the LEFT JOIN.

FossilOrigin-Name: 1838a59c8a1c151bd6fc822b0ffef661803cf0e4704c917e74a04567b81740b9

manifest

@@ -1,5 +1,5 @@
-C Make\sUSE_FULLWARN=1\sthe\sdefault\sfor\sMSVC\sand\sfix\sharmless\scompiler\swarnings.
-D 2017-04-14T14:50:34.600
+C An\sinitial\sattempt\sto\soptimize\sVIEWs\sthat\soccur\sas\sthe\sright\soperand\sof\sa\nLEFT\sJOIN.\s\sThis\sparticular\scheck-in\sdoes\snot\swork\scorrectly\sbecause\sit\sdoes\nnot\sdeal\swith\sthe\scase\sof\scolumns\sin\sthe\sVIEW\sthat\sreturn\snon-NULL\seven\swhen\nall\scolumns\sin\sthe\stable\sof\sthe\sVIEW\sare\sNULL\sbecause\sof\sthe\sLEFT\sJOIN.
+D 2017-04-14T17:18:45.647
 F Makefile.in 1cc758ce3374a32425e4d130c2fe7b026b20de5b8843243de75f087c0a2661fb
 F Makefile.linux-gcc 7bc79876b875010e8c8f9502eb935ca92aa3c434
 F Makefile.msc 6a8c838220f7c00820e1fc0ac1bccaaa8e5676067e1dbfa1bafa7a4ffecf8ae6
@@ -401,7 +401,7 @@ F src/printf.c 8757834f1b54dae512fb25eb1acc8e94a0d15dd2290b58f2563f65973265adb2
 F src/random.c 80f5d666f23feb3e6665a6ce04c7197212a88384
 F src/resolve.c 3e518b962d932a997fae373366880fc028c75706
 F src/rowset.c 7b7e7e479212e65b723bf40128c7b36dc5afdfac
-F src/select.c 4588dcfb0fa430012247a209ba08e17904dd32ec7690e9cb6c85e0ef012b0518
+F src/select.c 52b29bdeaa4cf317a981b6e8d0a15ec9c16372a738ede51c2a285c03fc69d036
 F src/shell.c 70f4957b988572315e97c56941fdc81fd35907fee36b7b2e7be5ec4c7e9d065d
 F src/sqlite.h.in 40233103e3e4e10f8a63523498d0259d232e42aba478e2d3fb914799185aced6
 F src/sqlite3.rc 5121c9e10c3964d5755191c80dd1180c122fc3a8
@@ -1571,7 +1571,10 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 5159cb8f2bcfb1f7114786ba23082d2b91a26e3a7ddfae75f8bd362792949d5e
-R 3270eeed59983be191492d424a0b906b
-U mistachkin
-Z edc8ddb73a9c8e57e014e4878b2c6186
+P 6bf673767b8e5cedef1acff795cbe524fab8db2525c06424db4e038934a33936
+R 8dfa3ca7454fc6936bedb04ccf47ee12
+T *branch * left-join-view
+T *sym-left-join-view *
+T -sym-trunk *
+U drh
+Z 5ef873863ca7134987a034a627c04882

manifest.uuid

@@ -1 +1 @@
-6bf673767b8e5cedef1acff795cbe524fab8db2525c06424db4e038934a33936
+1838a59c8a1c151bd6fc822b0ffef661803cf0e4704c917e74a04567b81740b9

src/select.c

@@ -3154,6 +3154,7 @@ static int multiSelectOrderBy(
 typedef struct SubstContext {
   Parse *pParse;            /* The parsing context */
   int iTable;               /* Replace references to this table */
+  int iNewTable;            /* New table number, or -1 */
   ExprList *pEList;         /* Replacement expressions */
 } SubstContext;
 
@@ -3179,6 +3180,9 @@ static Expr *substExpr(
   Expr *pExpr            /* Expr in which substitution occurs */
 ){
   if( pExpr==0 ) return 0;
+  if( ExprHasProperty(pExpr, EP_FromJoin) && pExpr->iRightJoinTable==pSubst->iTable ){
+    pExpr->iRightJoinTable = pSubst->iNewTable;
+  }
   if( pExpr->op==TK_COLUMN && pExpr->iTable==pSubst->iTable ){
     if( pExpr->iColumn<0 ){
       pExpr->op = TK_NULL;
@@ -3285,7 +3289,13 @@ static void substSelect(
 **        due to ticket [2f7170d73bf9abf80] from 2015-02-09.)
 **
 **   (3)  The subquery is not the right operand of a left outer join
-**        (Originally ticket #306.  Strengthened by ticket #3300)
+**        or (3b) the subquery is not a join and (3c) does not contain any
+**        result-set columns that could be non-NULL even if columns of the
+**        subquery table are NULL.
+**        to be more restrictive - disallowing subquery if it was the
+**        right operand of a left join.  See tickets #306 and #3300. Relaxed
+**        to allow simple subqueries as the right operand of a left join
+**        on 2017-04-14.)
 **
 **   (4)  The subquery is not DISTINCT.
 **
@@ -3403,6 +3413,8 @@ static int flattenSubquery(
   SrcList *pSubSrc;   /* The FROM clause of the subquery */
   ExprList *pList;    /* The result set of the outer query */
   int iParent;        /* VDBE cursor number of the pSub result set temp table */
+  int iNewParent = -1;/* Replacement table for iParent */
+  int isLeftJoin = 0; /* True if pSub is the right side of a LEFT JOIN */    
   int i;              /* Loop counter */
   Expr *pWhere;                    /* The WHERE clause */
   struct SrcList_item *pSubitem;   /* The subquery */
@@ -3429,7 +3441,7 @@ static int flattenSubquery(
       return 0;                                          /* Restriction (2b)  */
     }
   }
-    
+
   pSubSrc = pSub->pSrc;
   assert( pSubSrc );
   /* Prior to version 3.1.2, when LIMIT and OFFSET had to be simple constants,
@@ -3467,10 +3479,9 @@ static int flattenSubquery(
     return 0; /* Restriction (23) */
   }
 
-  /* OBSOLETE COMMENT 1:
-  ** Restriction 3:  If the subquery is a join, make sure the subquery is 
-  ** not used as the right operand of an outer join.  Examples of why this
-  ** is not allowed:
+  /*
+  ** If the subquery is the right operand of a LEFT JOIN, then the
+  ** subquery may not be a join itself.  Example of why this is not allowed:
   **
   **         t1 LEFT OUTER JOIN (t2 JOIN t3)
   **
@@ -3480,27 +3491,15 @@ static int flattenSubquery(
   **
   ** which is not at all the same thing.
   **
-  ** OBSOLETE COMMENT 2:
-  ** Restriction 12:  If the subquery is the right operand of a left outer
-  ** join, make sure the subquery has no WHERE clause.
-  ** An examples of why this is not allowed:
-  **
-  **         t1 LEFT OUTER JOIN (SELECT * FROM t2 WHERE t2.x>0)
-  **
-  ** If we flatten the above, we would get
-  **
-  **         (t1 LEFT OUTER JOIN t2) WHERE t2.x>0
-  **
-  ** But the t2.x>0 test will always fail on a NULL row of t2, which
-  ** effectively converts the OUTER JOIN into an INNER JOIN.
-  **
-  ** THIS OVERRIDES OBSOLETE COMMENTS 1 AND 2 ABOVE:
-  ** Ticket #3300 shows that flattening the right term of a LEFT JOIN
-  ** is fraught with danger.  Best to avoid the whole thing.  If the
-  ** subquery is the right term of a LEFT JOIN, then do not flatten.
+  ** See also tickets #306 and #3300.
   */
   if( (pSubitem->fg.jointype & JT_OUTER)!=0 ){
-    return 0;
+    isLeftJoin = 1;
+    if( pSubSrc->nSrc>1 ){
+      return 0; /* Restriction (3b) */
+    }
+    /* TBD: Deal with the case of result-set expressions that are non-NULL even
+    ** when the RHS table of a LEFT JOIN is NULL. */
   }
 
   /* Restriction 17: If the sub-query is a compound SELECT, then it must
@@ -3709,6 +3708,7 @@ static int flattenSubquery(
       sqlite3IdListDelete(db, pSrc->a[i+iFrom].pUsing);
       assert( pSrc->a[i+iFrom].fg.isTabFunc==0 );
       pSrc->a[i+iFrom] = pSubSrc->a[i];
+      iNewParent = pSubSrc->a[i].iCursor;
       memset(&pSubSrc->a[i], 0, sizeof(pSubSrc->a[i]));
     }
     pSrc->a[iFrom].fg.jointype = jointype;
@@ -3754,6 +3754,9 @@ static int flattenSubquery(
       pSub->pOrderBy = 0;
     }
     pWhere = sqlite3ExprDup(db, pSub->pWhere, 0);
+    if( isLeftJoin ){
+      setJoinExpr(pWhere, iNewParent);
+    }
     if( subqueryIsAgg ){
       assert( pParent->pHaving==0 );
       pParent->pHaving = pParent->pWhere;
@@ -3770,6 +3773,7 @@ static int flattenSubquery(
       SubstContext x;
       x.pParse = pParse;
       x.iTable = iParent;
+      x.iNewTable = iNewParent;
       x.pEList = pSub->pEList;
       substSelect(&x, pParent, 0);
     }
@@ -3878,6 +3882,7 @@ static int pushDownWhereTerms(
       pNew = sqlite3ExprDup(pParse->db, pWhere, 0);
       x.pParse = pParse;
       x.iTable = iCursor;
+      x.iNewTable = iCursor;
       x.pEList = pSubq->pEList;
       pNew = substExpr(&x, pNew);
       pSubq->pWhere = sqlite3ExprAnd(pParse->db, pSubq->pWhere, pNew);