Do not compute unused result columns of subqueries. This optimization will

potentially resolve the performance optimization request of
[baa5bb76c35a124c|ticket baa5bb76c35a124c].

FossilOrigin-Name: 0c21b6a5f8547cddc55f7ec27643dbb89e93c0aed23edc9f152be13d9846dd73

diff --cc manifest
index 868f09d845c9f5b6635db074cbf2f7a9037e8c15,38e7b2731d9d772cdcacbe90f8b6e702282e414b..b51827882f4e8c8eab9bd02ec00297737a579a8a
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,12 -1,13 +1,12 @@@
- C Update\stest\scases\sso\sthat\sthey\swork\swith\sTCL\s8.7\sand\slater.
- D 2023-02-16T01:21:51.925
 -C Fix\sthe\ssubquery\sresult\scolumn\sNULL-ifier\sso\sthat\sit\scorrectly\shandles\nsubquery\scolumns\spast\sthe\s63rd\scolumn.
 -D 2023-02-15T19:53:08.659
++C Do\snot\scompute\sunused\sresult\scolumns\sof\ssubqueries.\s\sThis\soptimization\swill\npotentially\sresolve\sthe\sperformance\soptimization\srequest\sof\n[baa5bb76c35a124c|ticket\sbaa5bb76c35a124c].
++D 2023-02-16T01:29:51.992
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  F sqlite3.1 fc7ad8990fc8409983309bb80de8c811a7506786
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 +F src/status.c 46e7aec11f79dad50965a5ca5fa9de009f7d6bde08be2156f1538a0a296d4d0e
 +F src/table.c b46ad567748f24a326d9de40e5b9659f96ffff34
 +F src/tclsqlite.c e72862a271348d779672b45a730c33fd0c535e630ff927e8ce4a0c908d1d28c6
 +F src/test1.c 5390e5afb31fed61f72d0be0cb1b322d198a6e03fc13ff245ae76d17b4dcf2e9
 +F src/test2.c 3efb99ab7f1fc8d154933e02ae1378bac9637da5
  F src/test3.c 61798bb0d38b915067a8c8e03f5a534b431181f802659a6616f9b4ff7d872644
 -F src/test4.c 4533b76419e7feb41b40582554663ed3cd77aaa54e135cf76b3205098cd6e664
 +F src/test4.c 18ec393bb4d0ad1de729f0b94da7267270f3d8e6
  F src/test5.c 328aae2c010c57a9829d255dc099d6899311672d
 -F src/test6.c ae73a3a42bbc982fb9e301b84d30bda65a307be48c6dff20aba1461e17a9b0ce
 -F src/test8.c 0c856d6ff6b0d2ff6696addc467a15ed17c6910f14475302cd5b3b4e54406161
 +F src/test6.c e8d839fbc552ce044bec8234561a2d5b8819b48e29548ad0ba400471697946a8
 +F src/test7.c 5612e9aecf934d6df7bba6ce861fdf5ba5456010
 +F src/test8.c 3f7d0cc4e12e06832ba3db4455cb16867ccadafa602eb6ff5fcf097bffce56ed
  F src/test9.c 12e5ba554d2d1cbe0158f6ab3f7ffcd7a86ee4e5
  F src/test_async.c 195ab49da082053fdb0f949c114b806a49ca770a
  F src/test_autoext.c 915d245e736652a219a907909bb6710f0d587871
@@@ -1779,8 -2045,8 +1779,8 @@@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a9
  F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
  F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
  F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
- P a7cbf30808e2c0c25aecd30c921b45a9888aff1a53f32350c5fc0196fb55b238
- R 8c04d000120f3201c20cbb96dbedc183
 -P 5dec3cc0225296a043d17f73126d477d90a604f82b3180628176d8f950adbce8
 -R d380059ff4fc01bc81597a53152668a1
++P e9b762de0e8c505f2334d931a5b6db862733e99bc12025ebf9d8882c696959fd 77b220a7240425fa83e142ceef78505208e6e38d797070e146b9f7d255f753c7
++R 87a6dbc349841006e9598dfb4e1e39bd
  U drh
- Z e19c4a4e958d5af50a94c7b8637d662f
 -Z d76a4bd1d7943a5eace66bda083496dd
++Z dffd8acc783ccd53f09f735ed943e784
  # Remove this line to create a well-formed Fossil manifest.

diff --cc manifest.uuid
index 1731d4bf84c1b124c22914db9909d81947b1c41d,149e343a10a4cd887441aee8f21ae312cc118573..23c3e50749ce9d6b26858ab8100594b9d3e12910
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- e9b762de0e8c505f2334d931a5b6db862733e99bc12025ebf9d8882c696959fd
 -77b220a7240425fa83e142ceef78505208e6e38d797070e146b9f7d255f753c7
++0c21b6a5f8547cddc55f7ec27643dbb89e93c0aed23edc9f152be13d9846dd73

diff --cc src/select.c
index b4e969640ea3b60ce9dbcc79c29e622364d0781d,dc187dfa6ae202e01eb13194b5dce239e9cd9673..fa5afab1364c3c9d40c2cf872c062bef83f5fdf2
--- 1/src/select.c
--- 2/src/select.c
+++ b/src/select.c
@@@ -4377,6 -5235,57 +4377,57 @@@ static int pushDownWhereTerms
  }
  #endif /* !defined(SQLITE_OMIT_SUBQUERY) || !defined(SQLITE_OMIT_VIEW) */
  
 -static int disableUnusedSubqueryResultColumns(SrcItem *pItem){
+ /*
+ ** 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.
+ */
 -    Bitmask m = j<BMS-1 ? MASKBIT(j) : TOPBIT;
++static int disableUnusedSubqueryResultColumns(struct SrcList_item *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;
 -      pX->selFlags |= SF_PushDown;
++    Bitmask m = j<BMS-1 ? MASKBIT(j) : (((Bitmask)1)<<(BMS-1));
+     if( (m & pItem->colUsed)!=0 ) continue;
+     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. 
@@@ -5870,10 -7379,25 +5921,24 @@@ int sqlite3Select
          sqlite3TreeViewSelect(0, p, 0);
        }
  #endif
 -      assert( pItem->pSelect && (pItem->pSelect->selFlags & SF_PushDown)!=0 );
      }else{
 -      TREETRACE(0x4000,pParse,p,("Push-down not possible\n"));
 +      SELECTTRACE(0x100,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;