Details
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Bug
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Status: Open (View Workflow)
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Major
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Resolution: Unresolved
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10.1
Description
Current EXPLAIN FORMAT=JSON output doesn't make it clear whether the subquery is correlated or not.
create table t0 (a int);
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INSERT INTO t0 VALUES (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
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create table t1 (a int, b int);
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insert into t1 select a,a from t0;
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MariaDB produces:
EXPLAIN: {
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"query_block": {
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"select_id": 1,
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"table": {
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"update": 1,
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"table_name": "t1",
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"access_type": "ALL",
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"rows": 10,
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"attached_condition": "(t1.a < 5)"
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},
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"subqueries": [
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{
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"query_block": {
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"select_id": 2,
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"table": {
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"table_name": "t0",
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"access_type": "ALL",
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"rows": 10,
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"filtered": 100,
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"attached_condition": "(t0.a < t1.b)"
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}
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}
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}
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]
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}
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}
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MySQL produces:
EXPLAIN: {
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"query_block": {
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"select_id": 1,
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"table": {
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"update": true,
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"table_name": "t1",
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"access_type": "ALL",
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"rows": 10,
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"filtered": 100,
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"attached_condition": "(`test`.`t1`.`a` < 5)"
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},
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"update_value_subqueries": [
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{
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"dependent": true,
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"cacheable": false,
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"query_block": {
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"select_id": 2,
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"table": {
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"table_name": "t0",
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"access_type": "ALL",
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"rows": 10,
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"filtered": 100,
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"attached_condition": "(`test`.`t0`.`a` < `test`.`t1`.`b`)"
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}
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}
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}
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]
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}
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}
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Note the
"dependent": true,
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"cacheable": false,
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these two look weird (does dependent == !cacheable always ?), but not being able to tell between correlated and uncorrelated subquery at all is not a solution.
I think there are two possible options (correlated/non-correlated).