From aa250db66aed4ed8f969d61f05bc374aa8c2c2e6 Mon Sep 17 00:00:00 2001
From: drh <>
Date: Wed, 2 Aug 2023 12:39:56 +0000
Subject: [PATCH] Simplify the select.c logic that enforces the optimization
 fence around a MATERIALIZED common table expression.  Do not allow the
 reverse_unordered_selects setting to affect a fenced CTE that itself has an
 ORDER BY clause.

FossilOrigin-Name: 165b9bda243777ae7fb6f51ee7ad58771c581b82ef9a70268eb9764e9e6eba6f
---
 manifest      | 14 +++++++-------
 manifest.uuid |  2 +-
 src/select.c  | 24 ++++++++++--------------
 src/where.c   | 21 ++++++++++++++++++++-
 4 files changed, 38 insertions(+), 23 deletions(-)

diff --git a/manifest b/manifest
index 228342f6be..043e5c2130 100644
--- a/manifest
+++ b/manifest
@@ -1,5 +1,5 @@
-C Never\sflatten\sa\sCTE\sthat\sis\slabeled\sMATERIALIZED.\s\sThis\sis\sreally\ssomething\nof\sa\sbug\sfix\sbecause\sthe\sdocumentation\ssays\sthat\sa\sMATERIALIZED\sCTE\sis\san\noptimization\sbarrier.
-D 2023-08-02T11:06:27.030
+C Simplify\sthe\sselect.c\slogic\sthat\senforces\sthe\soptimization\sfence\saround\na\sMATERIALIZED\scommon\stable\sexpression.\s\sDo\snot\sallow\sthe\nreverse_unordered_selects\ssetting\sto\saffect\sa\sfenced\sCTE\sthat\sitself\shas\nan\sORDER\sBY\sclause.
+D 2023-08-02T12:39:56.675
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -643,7 +643,7 @@ F src/printf.c e3ba080e2f409f9bfcc8d34724e6fc160e9c718dc92d0548f6b71b8b6f860ce2
 F src/random.c 606b00941a1d7dd09c381d3279a058d771f406c5213c9932bbd93d5587be4b9c
 F src/resolve.c 37953a5f36c60bea413c3c04efcd433b6177009f508ef2ace0494728912fe2e9
 F src/rowset.c 8432130e6c344b3401a8874c3cb49fefe6873fec593294de077afea2dce5ec97
-F src/select.c 10ff496138ffa5bf5c3d9c7808b7558cb9b398d12062c464037167748075a13f
+F src/select.c 8a8b7117bed5b7ce8122b611a9a7ff9504c523de3793d3a26661bf8ad81577ef
 F src/shell.c.in 694aaf751f00610381533d4a31c83d142cfc83ef91ef65e2aa6912ace7c39b40
 F src/sqlite.h.in 7b07a33d2af82ee974aa91e6294abce0282b2f4c5934b291d2fff961810dd867
 F src/sqlite3.rc 5121c9e10c3964d5755191c80dd1180c122fc3a8
@@ -728,7 +728,7 @@ F src/vxworks.h d2988f4e5a61a4dfe82c6524dd3d6e4f2ce3cdb9
 F src/wal.c dd843f619ac60d5dadab7109cf402432ba74dde0c301505fd1c202add07659e3
 F src/wal.h c3aa7825bfa2fe0d85bef2db94655f99870a285778baa36307c0a16da32b226a
 F src/walker.c 7c7ea0115345851c3da4e04e2e239a29983b61fb5b038b94eede6aba462640e2
-F src/where.c dbbd8588329e211fc797ee3d48cde21ad7a3ba0f9b135db9f10e5ba44690cf43
+F src/where.c d4240edf4c78b94e705f774509f0552ebcf08a1d2ee169fe14d66eccb4dc910a
 F src/whereInt.h c7d19902863beadec1d04e66aca39c0bcd60b74f05f0eaa7422c7005dfc5d51a
 F src/wherecode.c 5d77db30a2a3dd532492ae882de114edba2fae672622056b1c7fd61f5917a8f1
 F src/whereexpr.c dc5096eca5ed503999be3bdee8a90c51361289a678d396a220912e9cb73b3c00
@@ -2049,8 +2049,8 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P aa769ee7477171f26a091cadff98aa4f48da7752ade20b405a95c4525456f62e
-R 5d3359f6c6d5e2c6f797651e2c52272a
+P b7ef9796f548ce43e06673f86bbec68157aae5e4ee8451e1d87ee5f70af1bb27
+R b2e3c3d0a4d53532b578742c3846e988
 U drh
-Z d3d190ae3372df83f9be614d98609288
+Z 94c05c39e8f919e5e81448014dcce915
 # Remove this line to create a well-formed Fossil manifest.
diff --git a/manifest.uuid b/manifest.uuid
index 1ef995c32e..9736ae7713 100644
--- a/manifest.uuid
+++ b/manifest.uuid
@@ -1 +1 @@
-b7ef9796f548ce43e06673f86bbec68157aae5e4ee8451e1d87ee5f70af1bb27
\ No newline at end of file
+165b9bda243777ae7fb6f51ee7ad58771c581b82ef9a70268eb9764e9e6eba6f
\ No newline at end of file
diff --git a/src/select.c b/src/select.c
index 7a3ba4a6fa..482d7283c3 100644
--- a/src/select.c
+++ b/src/select.c
@@ -7285,6 +7285,14 @@ int sqlite3Select(
       goto select_end;
     }
 
+    /* Do not attempt the usual optimizations (flattening and ORDER BY
+    ** elimination) on a MATERIALIZED common table expression because
+    ** a MATERIALIZED common table expression is an optimization fence.
+    */
+    if( pItem->fg.isCte && pItem->u2.pCteUse->eM10d==M10d_Yes ){
+      continue;
+    }
+
     /* Do not try to flatten an aggregate subquery.
     **
     ** Flattening an aggregate subquery is only possible if the outer query
@@ -7315,21 +7323,14 @@ int sqlite3Select(
     **            (b)  The subquery is part of a join
     **          See forum post 062d576715d277c8
     **
-    ** The above cases might get an incorrect result if ORDER BY is omitted.
-    ** The following constraints are not required for correct answers, but are
-    ** included in order to give developers more control over when a sort
-    ** occurs:
-    **
-    **    (6)   The subquery is really a MATERIALIZED CTE
-    **    (7)   The OmitOrderBy optimization is disabled
+    ** Also retain the ORDER BY if the OmitOrderBy optimization is disabled.
     */
     if( pSub->pOrderBy!=0
      && (p->pOrderBy!=0 || pTabList->nSrc>1)      /* Condition (5) */
      && pSub->pLimit==0                           /* Condition (1) */
      && (pSub->selFlags & SF_OrderByReqd)==0      /* Condition (2) */
      && (p->selFlags & SF_OrderByReqd)==0         /* Condition (3) and (4) */
-     && (pItem->fg.isCte==0 || pItem->u2.pCteUse->eM10d!=M10d_Yes) /* (6) */
-     && OptimizationEnabled(db, SQLITE_OmitOrderBy)                /* (7) */
+     && OptimizationEnabled(db, SQLITE_OmitOrderBy)
     ){
       TREETRACE(0x800,pParse,p,
                 ("omit superfluous ORDER BY on %r FROM-clause subquery\n",i+1));
@@ -7366,11 +7367,6 @@ int sqlite3Select(
       continue;
     }
 
-    /* Do not flatten a MATERIALIZED CTE */
-    if( pItem->fg.isCte && pItem->u2.pCteUse->eM10d==M10d_Yes ){
-      continue;
-    }
-
     if( flattenSubquery(pParse, p, i, isAgg) ){
       if( pParse->nErr ) goto select_end;
       /* This subquery can be absorbed into its parent. */
diff --git a/src/where.c b/src/where.c
index c75834ff59..9992b2053f 100644
--- a/src/where.c
+++ b/src/where.c
@@ -6122,8 +6122,27 @@ WhereInfo *sqlite3WhereBegin(
        if( db->mallocFailed ) goto whereBeginError;
     }
   }
+  assert( pWInfo->pTabList!=0 );
   if( pWInfo->pOrderBy==0 && (db->flags & SQLITE_ReverseOrder)!=0 ){
-     pWInfo->revMask = ALLBITS;
+    for(ii=0; ii<pWInfo->pTabList->nSrc; ii++){
+      /* The PRAGMA reverse_unordered_selects=ON setting (also accessible
+      ** using SQLITE_DBCONFIG_REVERSE_SCANORDER) means to reverse the scan
+      ** order for any table that is part of a query that does not have an
+      ** ORDER BY clause.
+      **
+      ** Except, do not reverse the output from MATERIALIZED common table
+      ** expression that has an internal ORDER BY clause, because a
+      ** MATERIALIZED common table expression is an optimization fence.
+      */
+      SrcItem *pItem = &pWInfo->pTabList->a[ii];
+      if( !pItem->fg.isCte
+       || pItem->u2.pCteUse->eM10d!=M10d_Yes
+       || NEVER(pItem->pSelect==0)
+       || pItem->pSelect->pOrderBy==0
+      ){
+        pWInfo->revMask |= MASKBIT(ii);
+      }
+    }
   }
   if( pParse->nErr ){
     goto whereBeginError;