Do not reduce subquery output row count estimates due to DISTINCT until

after the decision of whether or not to use an index for ORDER BY has been
made.

FossilOrigin-Name: 27390051e86ad86fb35219329d359be9e83073f59782631af7fc519225e10565

manifest

@@ -1,5 +1,5 @@
-C Experimental:\s\sAssume\sthat\sa\sDISTINCT\sclause\son\sa\ssubquery\sreduces\sthe\snumber\nof\srows\sreturned\sby\sthat\ssubquery\sby\sa\sfactor\sof\s8.
-D 2023-09-15T16:15:33.055
+C Do\snot\sreduce\ssubquery\soutput\srow\scount\sestimates\sdue\sto\sDISTINCT\suntil\nafter\sthe\sdecision\sof\swhether\sor\snot\sto\suse\san\sindex\sfor\sORDER\sBY\shas\sbeen\nmade.
+D 2023-09-15T19:00:47.376
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -795,7 +795,7 @@ F src/vxworks.h d2988f4e5a61a4dfe82c6524dd3d6e4f2ce3cdb9
 F src/wal.c 01e051a1e713d9eabdb25df38602837cec8f4c2cae448ce2cf6accc87af903e9
 F src/wal.h ba252daaa94f889f4b2c17c027e823d9be47ce39da1d3799886bbd51f0490452
 F src/walker.c 7c7ea0115345851c3da4e04e2e239a29983b61fb5b038b94eede6aba462640e2
-F src/where.c 6cdf686c80cea6bf01983714e8fa291976072429dc3d5f9f0ee63340ff31c20a
+F src/where.c bd10b322a40abc2ebfba61fd1f3c434052f68cf991487c4ed1da9c9f94aa0e87
 F src/whereInt.h 4b38c5889514e3aead3f27d0ee9a26e47c3f150efc59e2a8b4e3bc8835e4d7a1
 F src/wherecode.c 5d77db30a2a3dd532492ae882de114edba2fae672622056b1c7fd61f5917a8f1
 F src/whereexpr.c dc5096eca5ed503999be3bdee8a90c51361289a678d396a220912e9cb73b3c00
@@ -1602,7 +1602,7 @@ F test/stmtvtab1.test 6873dfb24f8e79cbb5b799b95c2e4349060eb7a3b811982749a84b3594
 F test/strict1.test 4d2b492152b984fd7e8196d23eb88e2ccb0ef9e46ca2f96c2ce7147ceef9d168
 F test/strict2.test b22c7a98b5000aef937f1990776497f0e979b1a23bc4f63e2d53b00e59b20070
 F test/subjournal.test 8d4e2572c0ee9a15549f0d8e40863161295107e52f07a3e8012a2e1fdd093c49
-F test/subquery.test 3a1a5b600b8d4f504d2a2c61f33db820983dba94a0ef3e4aedca8f0165eaecb8
+F test/subquery.test 312c5d26304b0e93a56ba2cb9d4480b8a1c8217e3b2b8f8be2bfb0b2458ac3a7
 F test/subquery2.test 90cf944b9de8204569cf656028391e4af1ccc8c0cc02d4ef38ee3be8de1ffb12
 F test/subselect.test 0966aa8e720224dbd6a5e769a3ec2a723e332303
 F test/substr.test a673e3763e247e9b5e497a6cacbaf3da2bd8ec8921c0677145c109f2e633f36b
@@ -2121,8 +2121,8 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P d9625a9eafe88859c3255849ee750cc6322d5a13ddad45107f9b2085c20eff50
-R 4ef1ee11935ce0c2e4bd33d7caf58576
+P 5a940e47d501f23347eaf084f4820e064e93665d19c8938c80715b71006ede8b
+R 708ef68b1a5a2dad2e7d5b5c554e468e
 U drh
-Z 02839c7f0b57c458709ba0da92bf6fc7
+Z 3822ece14e8e4a55dc3118a267231484
 # Remove this line to create a well-formed Fossil manifest.

manifest.uuid

@@ -1 +1 @@
-5a940e47d501f23347eaf084f4820e064e93665d19c8938c80715b71006ede8b
+27390051e86ad86fb35219329d359be9e83073f59782631af7fc519225e10565

src/where.c

@@ -5353,15 +5353,6 @@ static int wherePathSolver(WhereInfo *pWInfo, LogEst nRowEst){
 
   pWInfo->nRowOut = pFrom->nRow;
 
-  /* TUNING:  Assume that a DISTINCT clause on a subquery reduces
-  ** the output size by a factor of 8 (LogEst -30)
-  */
-  if( (pWInfo->wctrlFlags & WHERE_WANT_DISTINCT)!=0
-   && pWInfo->nRowOut>30
-  ){
-    pWInfo->nRowOut -= 30;
-  }
-
   /* Free temporary memory and return success */
   sqlite3StackFreeNN(pParse->db, pSpace);
   return SQLITE_OK;
@@ -6141,6 +6132,18 @@ WhereInfo *sqlite3WhereBegin(
        wherePathSolver(pWInfo, pWInfo->nRowOut+1);
        if( db->mallocFailed ) goto whereBeginError;
     }
+
+    /* TUNING:  Assume that a DISTINCT clause on a subquery reduces
+    ** the output size by a factor of 8 (LogEst -30)
+    */
+    if( (pWInfo->wctrlFlags & WHERE_WANT_DISTINCT)!=0
+     && pWInfo->nRowOut>=40
+    ){
+      WHERETRACE(0x0080,("nRowOut reduced from %d to %d due to DISTINCT\n",
+                         pWInfo->nRowOut, pWInfo->nRowOut-30));
+      pWInfo->nRowOut -= 30;
+    }
+
   }
   assert( pWInfo->pTabList!=0 );
   if( pWInfo->pOrderBy==0 && (db->flags & SQLITE_ReverseOrder)!=0 ){

test/subquery.test

@@ -11,8 +11,6 @@
 # This file implements regression tests for SQLite library.  The
 # focus of this script is testing correlated subqueries
 #
-# $Id: subquery.test,v 1.17 2009/01/09 01:12:28 drh Exp $
-#
 
 set testdir [file dirname $argv0]
 source $testdir/tester.tcl
@@ -613,4 +611,45 @@ do_execsql_test subquery-9.4 {
   SELECT (SELECT DISTINCT x FROM t1 ORDER BY +x LIMIT 1 OFFSET 2) FROM t1;
 } {{} {} {} {}}
 
+# 2023-09-15
+# Query planner performance regression reported by private email 
+# on 2023-09-14, caused by VIEWSCAN optimization of check-in 609fbb94b8f01d67
+# from 2022-09-01.
+#
+reset_db
+do_execsql_test subquery-10.1 {
+  CREATE TABLE t1(aa TEXT, bb INT, cc TEXT);
+  CREATE INDEX x11 on t1(bb);
+  CREATE INDEX x12 on t1(aa);
+  CREATE TABLE t2(aa TEXT, xx INT);
+  ANALYZE sqlite_master;
+  INSERT INTO sqlite_stat1(tbl, idx, stat) VALUES('t1', 'x11', '156789 28');
+  INSERT INTO sqlite_stat1(tbl, idx, stat) VALUES('t1', 'x12', '156789 1');
+  ANALYZE sqlite_master;
+}
+do_eqp_test subquery-10.2 {
+  WITH v1(aa,cc,bb) AS (SELECT aa, cc, bb FROM t1 WHERE bb=12345),
+       v2(aa,mx)    AS (SELECT aa, max(xx) FROM t2 GROUP BY aa)
+  SELECT * FROM v1 JOIN v2 ON v1.aa=v2.aa;
+} {
+  QUERY PLAN
+  |--CO-ROUTINE v2
+  |  |--SCAN t2
+  |  `--USE TEMP B-TREE FOR GROUP BY
+  |--SEARCH t1 USING INDEX x11 (bb=?)
+  `--SEARCH v2 USING AUTOMATIC COVERING INDEX (aa=?)
+}
+# ^^^^^^^^^^^^^
+# Prior to the fix the incorrect (slow) plan caused by the
+# VIEWSCAN optimization was:
+#
+# QUERY PLAN
+# |--CO-ROUTINE v2
+# |  |--SCAN t2
+# |  `--USE TEMP B-TREE FOR GROUP BY
+# |--SCAN v2
+# `--SEARCH t1 USING INDEX x12 (aa=?)
+#
+
+
 finish_test