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ef966af801
(Variant 3) (commit in 11.4)
When a derived table has a GROUP BY clause:
SELECT ...
FROM (SELECT ... GROUP BY col1, col2) AS tbl
The optimizer would use inner join's output cardinality as an estimate
of derived table size, ignoring the fact that GROUP BY operation would
produce much fewer groups.
Add code to produce tighter bounds:
- The GROUP BY list is split into per-table lists. If GROUP BY list has
expressions that refer to multiple tables, we fall back to join output
cardinality.
- For each table, the first cardinality estimate is join_tab->read_records.
- Then, we try to get a tighter bound by using index statistics.
- If indexes do not cover all GROUP BY columns, we try to use per-column
EITS statistics.
213 lines
6.1 KiB
Text
213 lines
6.1 KiB
Text
#
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# MDEV-30877: Output cardinality for derived table ignores GROUP BY
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#
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create table t1 (
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groups_20 int,
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groups_20_2 int,
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b int,
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index (groups_20)
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);
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insert into t1 select seq/1000, seq/1000, seq from seq_1_to_20000;
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create table t2 (a int, b int, index(a));
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insert into t2 select seq, seq from seq_1_to_10;
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analyze table t1 persistent for all;
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Table Op Msg_type Msg_text
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test.t1 analyze status Engine-independent statistics collected
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test.t1 analyze status Table is already up to date
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set optimizer_trace=1;
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# Case 1: one indexed column
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explain
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select *
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from
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t2,
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(select count(*) cnt, groups_20 from t1
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group by groups_20) TBL
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where
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1;
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t2 ALL NULL NULL NULL NULL 10
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1 PRIMARY <derived2> ALL NULL NULL NULL NULL 20 Using join buffer (flat, BNL join)
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2 DERIVED t1 index NULL groups_20 5 NULL 20000 Using index
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select
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json_detailed(json_extract(trace, '$**.materialized_output_cardinality'))
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from
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information_schema.optimizer_trace;
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json_detailed(json_extract(trace, '$**.materialized_output_cardinality'))
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[
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{
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"join_output_cardinality": 20000,
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"estimation":
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[
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{
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"table": "t1",
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"steps":
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[
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{
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"index_name": "groups_20",
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"cardinality": 20.99999895
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}
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],
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"cardinality": 20.99999895
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}
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],
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"post_group_cardinality": 20.99999895
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}
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]
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# Case 2: one non- indexed column
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explain
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select *
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from
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t2,
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(select count(*) cnt, groups_20_2 from t1
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group by groups_20_2) TBL
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where
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1;
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t2 ALL NULL NULL NULL NULL 10
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1 PRIMARY <derived2> ALL NULL NULL NULL NULL 20 Using join buffer (flat, BNL join)
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2 DERIVED t1 ALL NULL NULL NULL NULL 20000 Using temporary; Using filesort
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select
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json_detailed(json_extract(trace, '$**.materialized_output_cardinality'))
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from
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information_schema.optimizer_trace;
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json_detailed(json_extract(trace, '$**.materialized_output_cardinality'))
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[
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{
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"join_output_cardinality": 20000,
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"estimation":
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[
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{
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"table": "t1",
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"steps":
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[
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{
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"column": "groups_20_2",
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"cardinality": 20.99999895
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}
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],
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"cardinality": 20.99999895
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}
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],
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"post_group_cardinality": 20.99999895
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}
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]
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# Case 4: one indexed column, multiple tables
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create table t3(c int);
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insert into t3 select seq from seq_1_to_10;
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explain
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select *
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from
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t2,
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(select count(*) cnt, groups_20 from t1,t3
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group by groups_20) TBL;
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t2 ALL NULL NULL NULL NULL 10
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1 PRIMARY <derived2> ALL NULL NULL NULL NULL 20 Using join buffer (flat, BNL join)
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2 DERIVED t3 ALL NULL NULL NULL NULL 10 Using temporary; Using filesort
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2 DERIVED t1 index NULL groups_20 5 NULL 20000 Using index; Using join buffer (flat, BNL join)
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select
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json_detailed(json_extract(trace, '$**.materialized_output_cardinality'))
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from
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information_schema.optimizer_trace;
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json_detailed(json_extract(trace, '$**.materialized_output_cardinality'))
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[
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{
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"join_output_cardinality": 200000,
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"estimation":
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[
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{
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"table": "t1",
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"steps":
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[
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{
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"index_name": "groups_20",
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"cardinality": 20.99999895
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}
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],
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"cardinality": 20.99999895
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}
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],
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"post_group_cardinality": 20.99999895
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}
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]
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# Case 5: group by two tables
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explain
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select *
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from
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t2,
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(select count(*) cnt, groups_20 from t1,t3
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group by groups_20, t3.c) TBL;
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t2 ALL NULL NULL NULL NULL 10
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1 PRIMARY <derived2> ALL NULL NULL NULL NULL 209 Using join buffer (flat, BNL join)
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2 DERIVED t3 ALL NULL NULL NULL NULL 10 Using temporary; Using filesort
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2 DERIVED t1 index NULL groups_20 5 NULL 20000 Using index; Using join buffer (flat, BNL join)
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select
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json_detailed(json_extract(trace, '$**.materialized_output_cardinality'))
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from
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information_schema.optimizer_trace;
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json_detailed(json_extract(trace, '$**.materialized_output_cardinality'))
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[
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{
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"join_output_cardinality": 200000,
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"estimation":
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[
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{
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"table": "t1",
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"steps":
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[
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{
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"index_name": "groups_20",
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"cardinality": 20.99999895
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}
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],
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"cardinality": 20.99999895
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},
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{
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"table": "t3",
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"steps":
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[],
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"cardinality": 10
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}
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],
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"post_group_cardinality": 209.9999895
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}
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]
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# Now, without an index
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explain
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select *
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from
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t2,
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(select count(*) cnt, groups_20 from t1 use index(),t3 group by groups_20) TBL;
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id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t2 ALL NULL NULL NULL NULL 10
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1 PRIMARY <derived2> ALL NULL NULL NULL NULL 20 Using join buffer (flat, BNL join)
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2 DERIVED t3 ALL NULL NULL NULL NULL 10 Using temporary; Using filesort
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2 DERIVED t1 ALL NULL NULL NULL NULL 20000 Using join buffer (flat, BNL join)
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select
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json_detailed(json_extract(trace, '$**.materialized_output_cardinality'))
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from
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information_schema.optimizer_trace;
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json_detailed(json_extract(trace, '$**.materialized_output_cardinality'))
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[
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{
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"join_output_cardinality": 200000,
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"estimation":
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[
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{
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"table": "t1",
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"steps":
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[
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{
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"column": "groups_20",
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"cardinality": 20.99999895
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}
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],
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"cardinality": 20.99999895
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}
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],
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"post_group_cardinality": 20.99999895
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}
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]
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set optimizer_trace=default;
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drop table t1, t2, t3;
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