From 0ff47e9e1b74e270648cac1ea72f9bcf4e35031d Mon Sep 17 00:00:00 2001
From: drh <drh@noemail.net>
Date: Tue, 15 Mar 2016 17:52:12 +0000
Subject: [PATCH] Implement FROM-clause subqueries as co-routines whenever they
 are guaranteed to be the outer-most loop of the join.

FossilOrigin-Name: c7bae50bdccb5bcf3bc22e8ac5bb6725ef13db39
---
 manifest          | 14 +++++++-------
 manifest.uuid     |  2 +-
 src/select.c      | 20 +++++++++++++++++---
 test/select4.test | 37 ++++++++++++++++++++++++++++++++++++-
 4 files changed, 61 insertions(+), 12 deletions(-)

diff --git a/manifest b/manifest
index 5b830ffdfa..61ece923e2 100644
--- a/manifest
+++ b/manifest
@@ -1,5 +1,5 @@
-C More\stest-case\schanges\sso\sthat\severything\sworks\swhen\sthe\sreserved_bytes\nvalue\sin\sthe\sheader\sis\snon-zero.
-D 2016-03-15T12:37:08.820
+C Implement\sFROM-clause\ssubqueries\sas\sco-routines\swhenever\sthey\sare\sguaranteed\nto\sbe\sthe\souter-most\sloop\sof\sthe\sjoin.
+D 2016-03-15T17:52:12.224
 F Makefile.in f53429fb2f313c099283659d0df6f20f932c861f
 F Makefile.linux-gcc 7bc79876b875010e8c8f9502eb935ca92aa3c434
 F Makefile.msc df0bf9ff7f8b3f4dd9fb4cc43f92fe58f6ec5c66
@@ -350,7 +350,7 @@ F src/printf.c 63e6fb12bbe702dd664dc3703776c090383a5a26
 F src/random.c ba2679f80ec82c4190062d756f22d0c358180696
 F src/resolve.c b8f7174e5f8c33c44ded3a25a973d0bb89228c20
 F src/rowset.c 9fe4b3ad7cc00944386bb600233d8f523de07a6e
-F src/select.c 137b31daa84d57d67847bf621bb54f3353e2077b
+F src/select.c 6dd2097bb158efe3b8d68683dcc3b4a49e907a34
 F src/shell.c 5e0ab1e708dc294330ccd8230536e1801f60822e
 F src/sqlite.h.in 0235586b3fb639e85998d495c90f007657fd82af
 F src/sqlite3.rc 5121c9e10c3964d5755191c80dd1180c122fc3a8
@@ -999,7 +999,7 @@ F test/securedel2.test 2d54c28e46eb1fd6902089958b20b1b056c6f1c5
 F test/select1.test be62204d2bd9a5a8a149e9974cfddce893d8f686
 F test/select2.test 352480e0e9c66eda9c3044e412abdf5be0215b56
 F test/select3.test 2ce595f8fb8e2ac10071d3b4e424cadd4634a054
-F test/select4.test 453631158540e5f685b81cac5b7e8bd8c6b4c5fc
+F test/select4.test d926792a5e4d88fef0ddcddeb45d27ce75f7296c
 F test/select5.test e758b8ef94f69b111df4cb819008856655dcd535
 F test/select6.test 39eac4a5c03650b2b473c532882273283ee8b7a0
 F test/select7.test 95e370c42d47c3c52377d05e9ffc01ccff7c1f61
@@ -1456,7 +1456,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 051c61ebae921b70f843c55aacf046a9922cc187
-R 1b522ee9926e242e3422d1eddaaf0fbf
+P 2fd095b14b03a508169bba710d2e8e2addf4b8f5
+R dae03fc18d1aaaa839efcf6d42490a7e
 U drh
-Z 3fc58440746cd83f06c07d7c16cdd74a
+Z 042547ad8e59422492ad781582901ed5
diff --git a/manifest.uuid b/manifest.uuid
index e76d63af73..b2a61f89c1 100644
--- a/manifest.uuid
+++ b/manifest.uuid
@@ -1 +1 @@
-2fd095b14b03a508169bba710d2e8e2addf4b8f5
\ No newline at end of file
+c7bae50bdccb5bcf3bc22e8ac5bb6725ef13db39
\ No newline at end of file
diff --git a/src/select.c b/src/select.c
index c9bc389b2f..a62581efc1 100644
--- a/src/select.c
+++ b/src/select.c
@@ -4970,10 +4970,24 @@ int sqlite3Select(
     }
 
     /* Generate code to implement the subquery
+    **
+    ** The subquery is implemented as a co-routine if all of these are true:
+    **   (1)  The subquery is guaranteed to be the outer loop (so that it
+    **        does not need to be computed more than once)
+    **   (2)  The ALL keyword after SELECT is omitted.  (Applications are
+    **        allowed to say "SELECT ALL" instead of just "SELECT" to disable
+    **        the use of co-routines.)
+    **   (3)  Co-routines are not disabled using sqlite3_test_control()
+    **        with SQLITE_TESTCTRL_OPTIMIZATIONS.
+    **
+    ** TODO: Are there other reasons beside (1) to use a co-routine
+    ** implementation?
     */
-    if( pTabList->nSrc==1
-     && (p->selFlags & SF_All)==0
-     && OptimizationEnabled(db, SQLITE_SubqCoroutine)
+    if( i==0
+     && (pTabList->nSrc==1
+            || (pTabList->a[1].fg.jointype&(JT_LEFT|JT_CROSS))!=0)  /* (1) */
+     && (p->selFlags & SF_All)==0                                   /* (2) */
+     && OptimizationEnabled(db, SQLITE_SubqCoroutine)               /* (3) */
     ){
       /* Implement a co-routine that will return a single row of the result
       ** set on each invocation.
diff --git a/test/select4.test b/test/select4.test
index be8d0e0ab1..1f29f29073 100644
--- a/test/select4.test
+++ b/test/select4.test
@@ -12,7 +12,6 @@
 # focus of this file is testing UNION, INTERSECT and EXCEPT operators
 # in SELECT statements.
 #
-# $Id: select4.test,v 1.30 2009/04/16 00:24:24 drh Exp $
 
 set testdir [file dirname $argv0]
 source $testdir/tester.tcl
@@ -936,4 +935,40 @@ do_execsql_test select4-15.1 {
    ORDER BY 1;
 } {1 33 456 2 33 789}
 
+# Enhancement (2016-03-15):  Use a co-routine for subqueries if the
+# subquery is guaranteed to be the outer-most query
+#
+do_execsql_test select4-16.1 {
+  DROP TABLE IF EXISTS t1;
+  CREATE TABLE t1(a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z,
+  PRIMARY KEY(a,b DESC)) WITHOUT ROWID;
+
+  WITH RECURSIVE c(x) AS (VALUES(1) UNION ALL SELECT x+1 FROM c WHERE x<100)
+  INSERT INTO t1(a,b,c,d)
+    SELECT x%10, x/10, x, printf('xyz%dabc',x) FROM c;
+
+  SELECT t3.c FROM 
+    (SELECT a,max(b) AS m FROM t1 WHERE a>=5 GROUP BY a) AS t2
+    JOIN t1 AS t3
+  WHERE t2.a=t3.a AND t2.m=t3.b
+  ORDER BY t3.a;
+} {95 96 97 98 99}
+do_execsql_test select4-16.2 {
+  SELECT t3.c FROM 
+    (SELECT a,max(b) AS m FROM t1 WHERE a>=5 GROUP BY a) AS t2
+    CROSS JOIN t1 AS t3
+  WHERE t2.a=t3.a AND t2.m=t3.b
+  ORDER BY t3.a;
+} {95 96 97 98 99}
+do_execsql_test select4-16.3 {
+  SELECT t3.c FROM 
+    (SELECT a,max(b) AS m FROM t1 WHERE a>=5 GROUP BY a) AS t2
+    LEFT JOIN t1 AS t3
+  WHERE t2.a=t3.a AND t2.m=t3.b
+  ORDER BY t3.a;
+} {95 96 97 98 99}
+
+
+
+
 finish_test