planner: generate index path as candidate when it is a single scan

cmd/explaintest/r/access_path_selection.result

@@ -15,9 +15,9 @@ IndexReader_6	3323.33	root	index:IndexScan_5
 └─IndexScan_5	3323.33	cop	table:access_path_selection, index:a, b, range:[-inf,3), keep order:false, stats:pseudo
 explain select a, b from access_path_selection where b < 3;
 id	count	task	operator info
-TableReader_7	3323.33	root	data:Selection_6
-└─Selection_6	3323.33	cop	lt(test.access_path_selection.b, 3)
-  └─TableScan_5	10000.00	cop	table:access_path_selection, range:[-inf,+inf], keep order:false, stats:pseudo
+IndexReader_13	3323.33	root	index:Selection_12
+└─Selection_12	3323.33	cop	lt(test.access_path_selection.b, 3)
+  └─IndexScan_11	10000.00	cop	table:access_path_selection, index:a, b, range:[NULL,+inf], keep order:false, stats:pseudo
 explain select a, b from access_path_selection where a < 3 and b < 3;
 id	count	task	operator info
 IndexReader_11	1104.45	root	index:Selection_10
@@ -36,3 +36,9 @@ StreamAgg_13	1.00	root	funcs:max(test.access_path_selection._tidb_rowid)
   └─TableReader_27	1.00	root	data:Limit_26
     └─Limit_26	1.00	cop	offset:0, count:1
       └─TableScan_25	1.25	cop	table:access_path_selection, range:[-inf,+inf], keep order:true, desc, stats:pseudo
+explain select count(1) from access_path_selection;
+id	count	task	operator info
+StreamAgg_28	1.00	root	funcs:count(col_0)
+└─IndexReader_29	1.00	root	index:StreamAgg_8
+  └─StreamAgg_8	1.00	cop	funcs:count(1)
+    └─IndexScan_25	10000.00	cop	table:access_path_selection, index:a, range:[NULL,+inf], keep order:false, stats:pseudo

cmd/explaintest/r/explain_easy_stats.result

@@ -65,8 +65,8 @@ explain select count(b.c2) from t1 a, t2 b where a.c1 = b.c2 group by a.c1;
 id	count	task	operator info
 Projection_11	1985.00	root	cast(join_agg_0)
 └─HashLeftJoin_19	1985.00	root	inner join, inner:HashAgg_23, equal:[eq(test.a.c1, test.b.c2)]
-  ├─TableReader_32	1999.00	root	data:TableScan_31
-  │ └─TableScan_31	1999.00	cop	table:a, range:[-inf,+inf], keep order:false
+  ├─IndexReader_34	1999.00	root	index:IndexScan_33
+  │ └─IndexScan_33	1999.00	cop	table:a, index:c2, range:[NULL,+inf], keep order:false
   └─HashAgg_23	1985.00	root	group by:test.b.c2, funcs:count(test.b.c2), firstrow(test.b.c2)
     └─TableReader_30	1985.00	root	data:Selection_29
       └─Selection_29	1985.00	cop	not(isnull(test.b.c2))
@@ -92,8 +92,8 @@ explain select c1 from t1 where c1 in (select c2 from t2);
 id	count	task	operator info
 Projection_9	1985.00	root	test.t1.c1
 └─HashLeftJoin_17	1985.00	root	inner join, inner:HashAgg_21, equal:[eq(test.t1.c1, test.t2.c2)]
-  ├─TableReader_30	1999.00	root	data:TableScan_29
-  │ └─TableScan_29	1999.00	cop	table:t1, range:[-inf,+inf], keep order:false
+  ├─IndexReader_32	1999.00	root	index:IndexScan_31
+  │ └─IndexScan_31	1999.00	cop	table:t1, index:c2, range:[NULL,+inf], keep order:false
   └─HashAgg_21	1985.00	root	group by:test.t2.c2, funcs:firstrow(test.t2.c2)
     └─TableReader_28	1985.00	root	data:Selection_27
       └─Selection_27	1985.00	cop	not(isnull(test.t2.c2))
@@ -104,15 +104,15 @@ MemTableScan_4	10000.00	root
 explain select c2 = (select c2 from t2 where t1.c1 = t2.c1 order by c1 limit 1) from t1;
 id	count	task	operator info
 Projection_12	1999.00	root	eq(test.t1.c2, test.t2.c2)
-└─Apply_14	1999.00	root	CARTESIAN left outer join, inner:Limit_21
-  ├─TableReader_16	1999.00	root	data:TableScan_15
-  │ └─TableScan_15	1999.00	cop	table:t1, range:[-inf,+inf], keep order:false
-  └─Limit_21	1.00	root	offset:0, count:1
-    └─Projection_41	1.00	root	test.t2.c1, test.t2.c2
-      └─IndexLookUp_40	1.00	root	
-        ├─Limit_39	1.00	cop	offset:0, count:1
-        │ └─IndexScan_37	1.25	cop	table:t2, index:c1, range: decided by [eq(test.t1.c1, test.t2.c1)], keep order:true
-        └─TableScan_38	1.00	cop	table:t2, keep order:false, stats:pseudo
+└─Apply_14	1999.00	root	CARTESIAN left outer join, inner:Limit_23
+  ├─IndexReader_18	1999.00	root	index:IndexScan_17
+  │ └─IndexScan_17	1999.00	cop	table:t1, index:c2, range:[NULL,+inf], keep order:false
+  └─Limit_23	1.00	root	offset:0, count:1
+    └─Projection_43	1.00	root	test.t2.c1, test.t2.c2
+      └─IndexLookUp_42	1.00	root	
+        ├─Limit_41	1.00	cop	offset:0, count:1
+        │ └─IndexScan_39	1.25	cop	table:t2, index:c1, range: decided by [eq(test.t1.c1, test.t2.c1)], keep order:true
+        └─TableScan_40	1.00	cop	table:t2, keep order:false, stats:pseudo
 explain select * from t1 order by c1 desc limit 1;
 id	count	task	operator info
 Limit_10	1.00	root	offset:0, count:1
@@ -123,12 +123,12 @@ set @@session.tidb_opt_insubq_to_join_and_agg=0;
 explain select 1 in (select c2 from t2) from t1;
 id	count	task	operator info
 Projection_6	1999.00	root	5_aux_0
-└─HashLeftJoin_7	1999.00	root	CARTESIAN left outer semi join, inner:TableReader_12
-  ├─TableReader_9	1999.00	root	data:TableScan_8
-  │ └─TableScan_8	1999.00	cop	table:t1, range:[-inf,+inf], keep order:false
-  └─TableReader_12	0.00	root	data:Selection_11
-    └─Selection_11	0.00	cop	eq(1, test.t2.c2)
-      └─TableScan_10	1985.00	cop	table:t2, range:[-inf,+inf], keep order:false
+└─HashLeftJoin_7	1999.00	root	CARTESIAN left outer semi join, inner:TableReader_14
+  ├─IndexReader_11	1999.00	root	index:IndexScan_10
+  │ └─IndexScan_10	1999.00	cop	table:t1, index:c2, range:[NULL,+inf], keep order:false
+  └─TableReader_14	0.00	root	data:Selection_13
+    └─Selection_13	0.00	cop	eq(1, test.t2.c2)
+      └─TableScan_12	1985.00	cop	table:t2, range:[-inf,+inf], keep order:false
 explain format="dot" select 1 in (select c2 from t2) from t1;
 dot contents
 
@@ -138,23 +138,23 @@ node [style=filled, color=lightgrey]
 color=black
 label = "root"
 "Projection_6" -> "HashLeftJoin_7"
-"HashLeftJoin_7" -> "TableReader_9"
-"HashLeftJoin_7" -> "TableReader_12"
+"HashLeftJoin_7" -> "IndexReader_11"
+"HashLeftJoin_7" -> "TableReader_14"
 }
-subgraph cluster8{
+subgraph cluster10{
 node [style=filled, color=lightgrey]
 color=black
 label = "cop"
-"TableScan_8"
+"IndexScan_10"
 }
-subgraph cluster11{
+subgraph cluster13{
 node [style=filled, color=lightgrey]
 color=black
 label = "cop"
-"Selection_11" -> "TableScan_10"
+"Selection_13" -> "TableScan_12"
 }
-"TableReader_9" -> "TableScan_8"
-"TableReader_12" -> "Selection_11"
+"IndexReader_11" -> "IndexScan_10"
+"TableReader_14" -> "Selection_13"
 }
 
 explain select * from index_prune WHERE a = 1010010404050976781 AND b = 26467085526790 LIMIT 1;

cmd/explaintest/r/generated_columns.result

@@ -32,9 +32,9 @@ IndexReader_6	3323.33	root	index:IndexScan_5
 └─IndexScan_5	3323.33	cop	table:sgc, index:a, b, range:[-inf,3), keep order:false, stats:pseudo
 EXPLAIN SELECT a, b from sgc where b < 3;
 id	count	task	operator info
-TableReader_7	3323.33	root	data:Selection_6
-└─Selection_6	3323.33	cop	lt(test.sgc.b, 3)
-  └─TableScan_5	10000.00	cop	table:sgc, range:[-inf,+inf], keep order:false, stats:pseudo
+IndexReader_13	3323.33	root	index:Selection_12
+└─Selection_12	3323.33	cop	lt(test.sgc.b, 3)
+  └─IndexScan_11	10000.00	cop	table:sgc, index:a, b, range:[NULL,+inf], keep order:false, stats:pseudo
 EXPLAIN SELECT a, b from sgc where a < 3 and b < 3;
 id	count	task	operator info
 IndexReader_11	1104.45	root	index:Selection_10

cmd/explaintest/r/index_join.result

@@ -51,5 +51,5 @@ Projection_8	10000.00	root	test.t1.a, test.t1.b
   │ ├─IndexScan_9	10.00	cop	table:t1, index:a, range: decided by [eq(test.t1.a, test.t2.a)], keep order:false, stats:pseudo
   │ └─TableScan_10	10.00	cop	table:t1, keep order:false, stats:pseudo
   └─StreamAgg_22	8000.00	root	group by:test.t2.a, funcs:firstrow(test.t2.a)
-    └─IndexReader_32	10000.00	root	index:IndexScan_31
-      └─IndexScan_31	10000.00	cop	table:t2, index:a, range:[NULL,+inf], keep order:true, stats:pseudo
+    └─IndexReader_35	10000.00	root	index:IndexScan_34
+      └─IndexScan_34	10000.00	cop	table:t2, index:a, range:[NULL,+inf], keep order:true, stats:pseudo