Enhance the EXISTS-to-JOIN optimization so that it works on EXISTS subqueries

that do not have a WHERE clause, and so that it works on nested EXISTS
subqueries.

FossilOrigin-Name: c1d5295724f9cf7f49e0786d28016eff2d268a2b670f934d24c76787626089db

manifest

@@ -1,5 +1,5 @@
-C Merge\sfixes\sfrom\strunk\sinto\sthe\sempty-table-optimizations\sbranch
-D 2025-07-04T14:24:18.375
+C Enhance\sthe\sEXISTS-to-JOIN\soptimization\sso\sthat\sit\sworks\son\sEXISTS\ssubqueries\nthat\sdo\snot\shave\sa\sWHERE\sclause,\sand\sso\sthat\sit\sworks\son\snested\sEXISTS\nsubqueries.
+D 2025-07-05T23:33:21.216
 F .fossil-settings/binary-glob 61195414528fb3ea9693577e1980230d78a1f8b0a54c78cf1b9b24d0a409ed6a x
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
@@ -785,7 +785,7 @@ F src/printf.c 71b6d3a0093bf23f473e25480ca0024e8962681506c75f4ffd3d343a3f0ab113
 F src/random.c 606b00941a1d7dd09c381d3279a058d771f406c5213c9932bbd93d5587be4b9c
 F src/resolve.c 5b14cad58bc21341fbaea76d7e781187559627a461745ece00c2655ba7c083ec
 F src/rowset.c 8432130e6c344b3401a8874c3cb49fefe6873fec593294de077afea2dce5ec97
-F src/select.c 6ce33294314f05898a35205566a7d7d5034f8a6f7ad9cb9e3804f4765a100290
+F src/select.c b1d4e1f21a32b4f33d3bf02c4167ac8da5aafb97210a0b13a9119d68183db37d
 F src/shell.c.in 73c0eeb7c265d59b99219d5aa055f412f07842088d8036b6d259927d85dd1bbf
 F src/sqlite.h.in 5c54f2461a1ea529bab8499148a2b238e2d4bb571d59e8ea5322d0c190abb693
 F src/sqlite3.rc 015537e6ac1eec6c7050e17b616c2ffe6f70fca241835a84a4f0d5937383c479
@@ -1140,7 +1140,7 @@ F test/exclusive.test 7ff63be7503990921838d5c9f77f6e33e68e48ed1a9d48cd28745bf650
 F test/exclusive2.test cd70b1d9c6fffd336f9795b711dcc5d9ceba133ad3f7001da3fda63615bdc91e
 F test/exec.test e949714dc127eaa5ecc7d723efec1ec27118fdd7
 F test/exists.test 79a75323c78f02bbe9c251ea502a092f9ef63dac
-F test/existsexpr.test d87e7ee394935f9b4a9a1a488f3faa55abd20dd85152efaf2034881a079c7ba1
+F test/existsexpr.test cefe49ffc2295281908e0e1dd6d289381d2c3bcbbabcf36cd799cedb0708c7e9
 F test/existsexpr2.test dc23e76389eff3d29f6488ff733012a3560cd67ec8cfaecbecd52cced5d5af11
 F test/existsfault.test ff41c11f3052c1bbd4f8dd557802310026253d67d7c4e3a180c16d2f0862973e
 F test/expr.test 4ada8eb822c45ef27a36851a258004d43c1e95e7c82585a1217e732084e4482c
@@ -2211,8 +2211,8 @@ F tool/version-info.c 3b36468a90faf1bbd59c65fd0eb66522d9f941eedd364fabccd7227350
 F tool/warnings-clang.sh bbf6a1e685e534c92ec2bfba5b1745f34fb6f0bc2a362850723a9ee87c1b31a7
 F tool/warnings.sh 1ad0169b022b280bcaaf94a7fa231591be96b514230ab5c98fbf15cd7df842dd
 F tool/win/sqlite.vsix deb315d026cc8400325c5863eef847784a219a2f
-P 872c41feddafcc21a02f1229ce017ceea9f8e309a4dd5b6e323477d67f975947 13af4acebe09b047756c22b800136cffaba532e7fcaa448a4edf4fedb94e9bbc
-R bc9dd605b1b6086398bb04180450d2c6
+P 6f98b16d210a9f5b6ca4b4599e3dab3263eddbae7c70ddbcabf988f4a1014e8b
+R 1a0b4833b41c9eafcebd2713b9567447
 U drh
-Z 9c89d6634e2f24fe30079554a53be2d7
+Z d76421db80a68ce3b2346aa4278aa4db
 # Remove this line to create a well-formed Fossil manifest.

manifest.uuid

@@ -1 +1 @@
-6f98b16d210a9f5b6ca4b4599e3dab3263eddbae7c70ddbcabf988f4a1014e8b
+c1d5295724f9cf7f49e0786d28016eff2d268a2b670f934d24c76787626089db

src/select.c

@@ -7418,11 +7418,17 @@ static int fromClauseTermCanBeCoroutine(
 **
 **    SELECT name FROM sailors AS S, reserves AS R
 **      WHERE S.sid = R.sid AND R.day = '2022-10-25';
+**
+** **Approximately**.  Really, we have to ensure that the FROM-clause term
+** that was formerly inside the EXISTS is only executed once.  This is handled
+** by setting the SrcItem.fg.fromExists flag, which then causes code in
+** the where.c file to exit the corresponding loop after the first successful
+** match (if any).
 */
 static SQLITE_NOINLINE void existsToJoin(
-  Parse *pParse,
-  Select *p,
-  Expr *pWhere
+  Parse *pParse,  /* Parsing context */
+  Select *p,      /* The SELECT statement being optimized */
+  Expr *pWhere    /* part of the WHERE clause currently being examined */
 ){
   if( pWhere 
    && !ExprHasProperty(pWhere, EP_OuterON|EP_InnerON) 
@@ -7437,9 +7443,9 @@ static SQLITE_NOINLINE void existsToJoin(
     }
     else if( pWhere->op==TK_EXISTS ){
       Select *pSub = pWhere->x.pSelect;
+      Expr *pSubWhere = pSub->pWhere;
       if( pSub->pSrc->nSrc==1
        && (pSub->selFlags & SF_Aggregate)==0
-       && pSub->pWhere
        && !pSub->pSrc->a[0].fg.isSubquery
       ){
         memset(pWhere, 0, sizeof(*pWhere));
@@ -7451,9 +7457,10 @@ static SQLITE_NOINLINE void existsToJoin(
         pSub->pSrc->a[0].fg.fromExists = 1;
         pSub->pSrc->a[0].fg.jointype |= JT_CROSS;
         p->pSrc = sqlite3SrcListAppendList(pParse, p->pSrc, pSub->pSrc);
-        p->pWhere = sqlite3PExpr(pParse, TK_AND, p->pWhere, pSub->pWhere);
-
-        pSub->pWhere = 0;
+        if( pSubWhere ){
+          p->pWhere = sqlite3PExpr(pParse, TK_AND, p->pWhere, pSubWhere);
+          pSub->pWhere = 0;
+        }
         pSub->pSrc = 0;
         sqlite3ParserAddCleanup(pParse, sqlite3SelectDeleteGeneric, pSub);
 #if TREETRACE_ENABLED
@@ -7463,6 +7470,7 @@ static SQLITE_NOINLINE void existsToJoin(
           sqlite3TreeViewSelect(0, p, 0);
         }
 #endif
+        existsToJoin(pParse, p, pSubWhere);
       }
     }
   }

test/existsexpr.test

@@ -193,7 +193,7 @@ do_subquery_test 3.7 1 {
   4 4 4
 }    
 
-do_subquery_test 3.8 1 {
+do_subquery_test 3.8 0 {
   SELECT * FROM y1 WHERE EXISTS ( SELECT a+1 FROM y2 )
 } {
   1 1 1