Performance improvements to the external merge-sorter. Keep content on an

in-memory linked lists rather than an ephemeral table prior to spilling to
disk.  Use the external merge-sorter to implement ORDER BY and GROUP BY
in addition to CREATE INDEX.

FossilOrigin-Name: 4c43e8b2d2c1d8dcba3cd1c3f2ec4e19ab419430

test/distinct.test

@@ -45,7 +45,7 @@ proc do_temptables_test {tn sql temptables} {
   uplevel [list do_test $tn [subst -novar {
     set ret ""
     db eval "EXPLAIN [set sql]" {
-      if {$opcode == "OpenEphemeral"} { 
+      if {$opcode == "OpenEphemeral" || $opcode == "SorterOpen"} { 
         if {$p5 != "10" && $p5!="00"} { error "p5 = $p5" }
         if {$p5 == "10"} {
           lappend ret hash

test/index4.test

@@ -108,5 +108,19 @@ do_execsql_test 1.8 {
   PRAGMA integrity_check
 } {ok}
 
+do_execsql_test 2.1 {
+  BEGIN;
+    CREATE TABLE t2(x);
+    INSERT INTO t2 VALUES(14);
+    INSERT INTO t2 VALUES(35);
+    INSERT INTO t2 VALUES(15);
+    INSERT INTO t2 VALUES(35);
+    INSERT INTO t2 VALUES(16);
+  COMMIT;
+}
+do_catchsql_test 2.2 {
+  CREATE UNIQUE INDEX i3 ON t2(x);
+} {1 {indexed columns are not unique}}
+
 
 finish_test

test/misc3.test

@@ -270,7 +270,7 @@ ifcapable {explain} {
       CREATE UNIQUE INDEX ex1i1 ON ex1(a);
       EXPLAIN REINDEX;
     }]
-    regexp { IsUnique \d+ \d+ \d+ \d+ } $x
+    regexp { SorterCompare \d+ \d+ \d+ } $x
   } {1}
   if {[regexp {16} [db one {PRAGMA encoding}]]} {
     do_test misc3-6.11-utf16 {