38 lines
1.5 KiB
Plaintext
38 lines
1.5 KiB
Plaintext
CREATE TABLE `access_path_selection` (
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`a` int,
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`b` int,
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KEY `IDX_a` (`a`),
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KEY `IDX_b` (`b`),
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KEY `IDX_ab` (`a`, `b`)
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);
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explain select a from access_path_selection where a < 3;
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# In this query, IDX_ab is better than IDX_a.
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# The reason is that we have to do double scan if we use IDX_a since it doesn't contain column b.
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explain select a, b from access_path_selection where a < 3;
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# In this query, IDX_ab can't be used, so IDX_b is the best.
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explain select a, b from access_path_selection where b < 3;
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explain select a, b from access_path_selection where a < 3 and b < 3;
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CREATE TABLE `outdated_statistics` (
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`a` int,
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`b` int,
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`c` int,
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INDEX idx_a(a),
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INDEX idx_ab(a,b)
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);
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insert into outdated_statistics values (2, 2, 2);
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insert into outdated_statistics values (3, 3, 3);
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insert into outdated_statistics values (4, 4, 4);
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analyze table outdated_statistics;
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insert into outdated_statistics values (1, 1, 1);
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insert into outdated_statistics values (1, 2, 2);
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insert into outdated_statistics values (1, 3, 3);
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# Only update idx_ab and leave idx_a outdated.
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# Then the estimated number of rows on idx_ab is 1 while it's 0 on idx_a.
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# But for this query, idx_ab is always better than idx_a,
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# because idx_a can't take column b into account while idx_ab can.
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# This wrong case can be solved by Skyline Pruning, so we may update its
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# result after Skyline Pruning is introduced.
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analyze table outdated_statistics index idx_ab;
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explain select * from outdated_statistics where a=1 and b=1 and c=1;
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