CREATE TABLE `access_path_selection` (
`a` int,
`b` int,
KEY `IDX_a` (`a`),
KEY `IDX_b` (`b`),
KEY `IDX_ab` (`a`, `b`)
);
explain select a from access_path_selection where a < 3;
id	count	task	operator info
IndexReader_6	3323.33	root	index:IndexScan_5
└─IndexScan_5	3323.33	cop	table:access_path_selection, index:a, range:[-inf,3), keep order:false, stats:pseudo
explain select a, b from access_path_selection where a < 3;
id	count	task	operator info
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
IndexLookUp_10	3323.33	root	
├─IndexScan_8	3323.33	cop	table:access_path_selection, index:b, range:[-inf,3), keep order:false, stats:pseudo
└─TableScan_9	3323.33	cop	table:access_path_selection, 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
└─Selection_10	1104.45	cop	lt(test.access_path_selection.b, 3)
  └─IndexScan_9	3323.33	cop	table:access_path_selection, index:a, b, range:[-inf,3), keep order:false, stats:pseudo
CREATE TABLE `outdated_statistics` (
`a` int,
`b` int,
`c` int,
INDEX idx_a(a),
INDEX idx_ab(a,b)
);
insert into outdated_statistics values (2, 2, 2);
insert into outdated_statistics values (3, 3, 3);
insert into outdated_statistics values (4, 4, 4);
analyze table outdated_statistics;
insert into outdated_statistics values (1, 1, 1);
insert into outdated_statistics values (1, 2, 2);
insert into outdated_statistics values (1, 3, 3);
analyze table outdated_statistics index idx_ab;
explain select * from outdated_statistics where a=1 and b=1 and c=1;
id	count	task	operator info
IndexLookUp_11	0.00	root	
├─IndexScan_8	1.00	cop	table:outdated_statistics, index:a, b, range:[1 1,1 1], keep order:false
└─Selection_10	0.00	cop	eq(test.outdated_statistics.c, 1)
  └─TableScan_9	1.00	cop	table:outdated_statistics, keep order:false
CREATE TABLE `unknown_correlation` (
id int,
a int,
PRIMARY KEY (`id`),
INDEX idx_a(a)
);
INSERT INTO unknown_correlation values (1, 1),(2, 1),(3, 1),(4, 1),(5, 1),(6, 1),(7, 1),(8, 1),(9, 1),(10, 1),(11, 1),(12, 1),(13, 1),(14, 1),(15, 1),(16, 1),(17, 1),(18, 1),(19, 1),(20, 2),(21, 2),(22, 2),(23, 2),(24, 2),(25, 2);
ANALYZE TABLE unknown_correlation;
EXPLAIN SELECT * FROM unknown_correlation WHERE a = 2 ORDER BY id limit 1;
id	count	task	operator info
Limit_11	1.00	root	offset:0, count:1
└─TableReader_24	1.00	root	data:Limit_23
  └─Limit_23	1.00	cop	offset:0, count:1
    └─Selection_21	1.00	cop	eq(test.unknown_correlation.a, 2)
      └─TableScan_20	4.17	cop	table:unknown_correlation, range:[-inf,+inf], keep order:true