Do not compute result columns of subqueries that are never used. Make those

columns NULL instead.  This optimization potentially resolves the enhancement
request described by [/tktview/baa5bb76c35a124c|ticket baa5bb76c35a124c].

FossilOrigin-Name: 5dec3cc0225296a043d17f73126d477d90a604f82b3180628176d8f950adbce8

manifest

@@ -1,5 +1,5 @@
-C Update\stestrunner.tcl\sto\srun\szipvfs\stest\sscripts\son\sunix.
-D 2023-02-14T18:09:40.825
+C Do\snot\scompute\sresult\scolumns\sof\ssubqueries\sthat\sare\snever\sused.\s\sMake\sthose\ncolumns\sNULL\sinstead.\s\sThis\soptimization\spotentially\sresolves\sthe\senhancement\nrequest\sdescribed\sby\s[/tktview/baa5bb76c35a124c|ticket\sbaa5bb76c35a124c].
+D 2023-02-15T17:53:17.086
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -621,7 +621,7 @@ F src/printf.c ff4b05e38bf928ff1b80d3dda4f977b10fe39ecbfe69c018224c7e5594fb2455
 F src/random.c 606b00941a1d7dd09c381d3279a058d771f406c5213c9932bbd93d5587be4b9c
 F src/resolve.c d62c5665279cc7485f9d45b5e20911cc7b19c203f268321a90d05d74f4725750
 F src/rowset.c ba9515a922af32abe1f7d39406b9d35730ed65efab9443dc5702693b60854c92
-F src/select.c c3ce1b49cca2c66c8c88fe7d9e1f3db23590deb4dd631619ad90e1e5d21bcf1f
+F src/select.c 49d9336c4108a765831b2483d17cf52e196caaf341cba95646a220ec70ae080f
 F src/shell.c.in 5cb750dc00bb8b20b020031fd1bb0cb8759a6e567c27aee6b0af31d187984985
 F src/sqlite.h.in c060f9e6f646fd817abf273c068666e079fc6e8467ea1d3e8b20f355f82722e9
 F src/sqlite3.rc 5121c9e10c3964d5755191c80dd1180c122fc3a8
@@ -1918,7 +1918,7 @@ F test/windowerr.tcl f5acd6fbc210d7b5546c0e879d157888455cd4a17a1d3f28f07c1c8a387
 F test/windowerr.test a8b752402109c15aa1c5efe1b93ccb0ce1ef84fa964ae1cd6684dd0b3cc1819b
 F test/windowfault.test 15094c1529424e62f798bc679e3fe9dfab6e8ba2f7dfe8c923b6248c31660a7c
 F test/windowpushd.test d8895d08870b7226f7693665bd292eb177e62ca06799184957b3ca7dc03067df
-F test/with1.test 9ad67fdeb2bbd808a5763c9060e214ea232f9b18d846ea3a59756dc38bdc3880
+F test/with1.test b93833890e5d2a368e78747f124503a0159aa029b98e9ed4795ebf630b2efd3d
 F test/with2.test a1df41b987198383b9b70bf5e5fda390582e46398653858dbc6ceb24253b28df
 F test/with3.test e7bf809bf75c1f44f98bca78bc331dbf542002c5227bf53c1261144db4e824c8
 F test/with4.test 257be66c0c67fee1defbbac0f685c3465e2cad037f21ce65f23f86084f198205
@@ -2045,8 +2045,11 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P ecdeef43b27412b0b0b09e09a62ad3a03836a3fc80f2070268090e7ca8f02712
-R 008d808935e4782501905c43f1b48b8b
-U dan
-Z 0a26d7ad99fa8ab0735a6c8903ff6006
+P e6c8e19ab0d6e7526d4596b75a45bb6becaf3c029690f7e75c016eac803c9990
+R 790a0333fb08765177eb98bd169ddbf7
+T *branch * omit-unused-subquery-columns
+T *sym-omit-unused-subquery-columns *
+T -sym-trunk *
+U drh
+Z e4cd68bda867ee6d3fbf83551561c523
 # Remove this line to create a well-formed Fossil manifest.

manifest.uuid

@@ -1 +1 @@
-e6c8e19ab0d6e7526d4596b75a45bb6becaf3c029690f7e75c016eac803c9990
+5dec3cc0225296a043d17f73126d477d90a604f82b3180628176d8f950adbce8

src/select.c

@@ -5235,6 +5235,57 @@ static int pushDownWhereTerms(
 }
 #endif /* !defined(SQLITE_OMIT_SUBQUERY) || !defined(SQLITE_OMIT_VIEW) */
 
+/*
+** Check to see if a subquery contains result-set columns that are
+** never used.  If it does, change the value of those result-set columns
+** to NULL so that they do not cause unnecessary work to compute.
+**
+** Return the number of column that were changed to NULL.
+*/
+static int disableUnusedSubqueryResultColumns(SrcItem *pItem){
+  int nCol;
+  Select *pSub;      /* The subquery to be simplified */
+  Select *pX;        /* For looping over compound elements of pSub */
+  Table *pTab;       /* The table that describes the subquery */
+  int j;             /* Column number */
+  int nChng = 0;     /* Number of columns converted to NULL */
+
+  assert( pItem!=0 );
+  assert( pItem->pTab!=0 );
+  pTab = pItem->pTab;
+  if( pTab->tabFlags & TF_Ephemeral ) return 0;
+  assert( pItem->pSelect!=0 );
+  pSub = pItem->pSelect;
+  assert( pSub->pEList->nExpr==pTab->nCol );
+  for(pX=pSub; pX; pX=pX->pPrior){
+    if( pX->pPrior && pX->op!=TK_ALL ){
+      /* This optimization does not work for compound subqueries that
+      ** use UNION, INTERSECT, or EXCEPT.  Only UNION ALL is allowed. */
+      return 0;
+    }
+    if( pX->pWin ){
+      /* This optimization does not work for subqueries that use window
+      ** functions. */
+      return 0;
+    }
+  }
+  nCol = pTab->nCol;
+  for(j=0; j<nCol; j++){
+    Select *pX;
+    if( (MASKBIT(j) & pItem->colUsed)!=0 ) continue;
+    if( MASKBIT(j)==0 ) break;
+    for(pX=pSub; pX; pX=pX->pPrior) {
+      Expr *pY = pX->pEList->a[j].pExpr;
+      if( pY->op==TK_NULL ) continue;
+      pY->op = TK_NULL;
+      pX->selFlags |= SF_PushDown;
+      nChng++;
+    }
+  }
+  return nChng;
+}
+
+
 /*
 ** The pFunc is the only aggregate function in the query.  Check to see
 ** if the query is a candidate for the min/max optimization. 
@@ -7333,6 +7384,20 @@ int sqlite3Select(
       TREETRACE(0x4000,pParse,p,("Push-down not possible\n"));
     }
 
+    /* Convert unused result columns of the subquery into simple NULL
+    ** expressions, to avoid unneeded searching and computation.
+    */
+    if( disableUnusedSubqueryResultColumns(pItem) ){
+#if TREETRACE_ENABLED
+      if( sqlite3TreeTrace & 0x4000 ){
+        TREETRACE(0x4000,pParse,p,
+            ("Change unused result columns to NULL for subquery %d:\n",
+             pSub->selId));
+        sqlite3TreeViewSelect(0, p, 0);
+      }
+#endif
+    }
+
     zSavedAuthContext = pParse->zAuthContext;
     pParse->zAuthContext = pItem->zName;
 

test/with1.test

@@ -1124,7 +1124,7 @@ do_execsql_test 24.1 {
   CREATE VIEW v1 AS SELECT max(a), min(b) FROM t1 GROUP BY c;
 }
 do_test 24.1 {
-  set program [db eval {EXPLAIN SELECT 1 FROM v1,v1,v1}]
+  set program [db eval {EXPLAIN SELECT * FROM v1 AS aa, v1 AS bb, v1 AS cc}]
   expr [lsearch $program OpenDup]>0
 } {1}
 do_execsql_test 24.2 {