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

FossilOrigin-Name: 0163b697dd560b6ad78ae20b3ae495a051680b0faf1633827119db381869351f

diff --cc manifest
index fbe7326545f175d48e5201d36c3b282faa2e2b25,38e7b2731d9d772cdcacbe90f8b6e702282e414b..2badceaa7a59e4763fed34e51ec4c110d99f7e1d
--- 1/manifest
--- 2/manifest
+++ b/manifest
@@@ -1,5 -1,5 +1,5 @@@
- C Improve\sthe\sability\sof\sthe\squery\splanner\sto\srecognize\scovering\sindexes\seven\non\stables\swith\smore\sthan\s63\scolumns\sand\swhere\sthe\sindex\sis\sover\scolumns\nbeyond\sthe\s63rd\scolumn.
- D 2022-10-24T13:50:04.830
 -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\sresult\scolumns\sof\ssubqueries\sthat\sare\snever\sused.\sMake\sthose\scolumns\sNULL\sinstead.\sThis\soptimization\spotentially\sresolves\sthe\senhancement\srequest\sdescribed\sby\s[baa5bb76c35a124c|ticket\sbaa5bb76c35a124c].
++D 2023-02-16T14:29:04.334
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 +F src/test8.c 3f7d0cc4e12e06832ba3db4455cb16867ccadafa602eb6ff5fcf097bffce56ed
  F src/test9.c 12e5ba554d2d1cbe0158f6ab3f7ffcd7a86ee4e5
  F src/test_async.c 195ab49da082053fdb0f949c114b806a49ca770a
  F src/test_autoext.c 915d245e736652a219a907909bb6710f0d587871
@@@ -1820,9 -2045,8 +1820,8 @@@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a9
  F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
  F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
  F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
- P 3da1032878bdc93f69b02926fb7243b31fe6b1a0ee93af68df52b203b0603dad
- Q +f058773e41495ddbae698f9e9a4f62b7003112ea8614dfad69471340058735e4
- R f77ec747567c3bc99f563b3d82675b4b
 -P 5dec3cc0225296a043d17f73126d477d90a604f82b3180628176d8f950adbce8
 -R d380059ff4fc01bc81597a53152668a1
++P 3d1992de4733d4e155f2d6d5268323410d1104ab56db50f08a2bb26bf87d926b 77b220a7240425fa83e142ceef78505208e6e38d797070e146b9f7d255f753c7
++R b3790c5c75be15bad2ca8d7c8cee6787
  U drh
- Z 75744969cee191ad91201d95cd6f1fac
 -Z d76a4bd1d7943a5eace66bda083496dd
++Z ce0e41ce709151b90d95961cc059402d
  # Remove this line to create a well-formed Fossil manifest.

diff --cc manifest.uuid
index 2dffe91d0ba4950d30b72a2d3822ee465e6d9875,149e343a10a4cd887441aee8f21ae312cc118573..a008c7610af262f202a3694f9ac7482649499600
--- 1/manifest.uuid
--- 2/manifest.uuid
+++ b/manifest.uuid
@@@ -1,1 -1,1 +1,1 @@@
- 3d1992de4733d4e155f2d6d5268323410d1104ab56db50f08a2bb26bf87d926b
 -77b220a7240425fa83e142ceef78505208e6e38d797070e146b9f7d255f753c7
++0163b697dd560b6ad78ae20b3ae495a051680b0faf1633827119db381869351f

diff --cc src/select.c
index 0c8ecd1c9e398a3cdce8677768c2ddbca852c21d,dc187dfa6ae202e01eb13194b5dce239e9cd9673..8ea78d3d201854ff043fc63878d1ce48ead93dcd
--- 1/src/select.c
--- 2/src/select.c
+++ b/src/select.c
@@@ -4375,6 -5235,57 +4375,57 @@@ 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.
+ */
 -      pX->selFlags |= SF_PushDown;
++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;
+     Bitmask m = j<BMS-1 ? MASKBIT(j) : TOPBIT;
+     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;