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[EXPLAIN] remove cross join from EXPLAIN ANALYZE #33613

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Merged
merged 2 commits into from
Sep 24, 2025
Merged

[EXPLAIN] remove cross join from EXPLAIN ANALYZE #33613

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17f6441
remove cross join from EXPLAIN ANALYZE
mgree Sep 17, 2025
095769b
drop unnecessary dump of old SQL output
mgree Sep 24, 2025
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31 changes: 1 addition & 30 deletions doc/user/content/sql/explain-analyze.md
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Original file line number Diff line number Diff line change
Expand Up @@ -313,35 +313,6 @@ EXPLAIN ANALYZE HINTS FOR INDEX wins_by_item AS SQL;
```

The results show the SQL that `EXPLAIN ANALYZE` would run to get the TopK hints
for the `wins_by_items` index:

```none
SELECT
repeat(' ', nesting * 2) || operator AS operator,
megsa.levels AS levels,
megsa.to_cut AS to_cut,
megsa.hint AS hint,
pg_size_pretty(savings) AS savings
FROM
mz_introspection.mz_lir_mapping AS mlm
JOIN
mz_introspection.mz_dataflow_global_ids AS mdgi
ON (mlm.global_id = mdgi.global_id)
LEFT JOIN
mz_introspection.mz_expected_group_size_advice AS megsa
ON
(
megsa.dataflow_id = mdgi.id
AND
mlm.operator_id_start <= megsa.region_id
AND
megsa.region_id < mlm.operator_id_end
)
JOIN
mz_introspection.mz_mappable_objects AS mo
ON (mlm.global_id = mo.global_id)
WHERE mo.name = 'materialize.public.wins_by_item'
ORDER BY mlm.lir_id DESC;
```
for the `wins_by_items` index.

[TopK hints]: /transform-data/idiomatic-materialize-sql/top-k/#query-hints-1
33 changes: 19 additions & 14 deletions src/sql/src/plan/statement/dml.rs
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Expand Up @@ -851,9 +851,10 @@ pub fn plan_explain_analyze(
SUM(mas.records) AS total_records,
CASE WHEN COUNT(DISTINCT mas.worker_id) <> 0 THEN SUM(mas.size) / COUNT(DISTINCT mas.worker_id) ELSE NULL END AS avg_memory,
CASE WHEN COUNT(DISTINCT mas.worker_id) <> 0 THEN SUM(mas.records) / COUNT(DISTINCT mas.worker_id) ELSE NULL END AS avg_records
FROM mz_introspection.mz_lir_mapping mlm
JOIN mz_introspection.mz_arrangement_sizes_per_worker mas
ON (mlm.operator_id_start <= mas.operator_id AND mas.operator_id < mlm.operator_id_end)
FROM mz_introspection.mz_lir_mapping mlm
CROSS JOIN generate_series((mlm.operator_id_start) :: int8, (mlm.operator_id_end - 1) :: int8) AS valid_id
JOIN mz_introspection.mz_arrangement_sizes_per_worker mas
ON (mas.operator_id = valid_id)
GROUP BY mlm.global_id, mlm.lir_id"#,
));
from.push("LEFT JOIN summary_memory sm USING (global_id, lir_id)");
Expand All @@ -867,9 +868,10 @@ GROUP BY mlm.global_id, mlm.lir_id"#,
mas.worker_id AS worker_id,
SUM(mas.size) AS worker_memory,
SUM(mas.records) AS worker_records
FROM mz_introspection.mz_lir_mapping mlm
JOIN mz_introspection.mz_arrangement_sizes_per_worker mas
ON (mlm.operator_id_start <= mas.operator_id AND mas.operator_id < mlm.operator_id_end)
FROM mz_introspection.mz_lir_mapping mlm
CROSS JOIN generate_series((mlm.operator_id_start) :: int8, (mlm.operator_id_end - 1) :: int8) AS valid_id
JOIN mz_introspection.mz_arrangement_sizes_per_worker mas
ON (mas.operator_id = valid_id)
GROUP BY mlm.global_id, mlm.lir_id, mas.worker_id"#,
));
from.push("LEFT JOIN per_worker_memory pwm USING (global_id, lir_id)");
Expand Down Expand Up @@ -907,9 +909,10 @@ GROUP BY mlm.global_id, mlm.lir_id, mas.worker_id"#,
mlm.lir_id AS lir_id,
SUM(mse.elapsed_ns) AS total_ns,
CASE WHEN COUNT(DISTINCT mse.worker_id) <> 0 THEN SUM(mse.elapsed_ns) / COUNT(DISTINCT mse.worker_id) ELSE NULL END AS avg_ns
FROM mz_introspection.mz_lir_mapping mlm
JOIN mz_introspection.mz_scheduling_elapsed_per_worker mse
ON (mlm.operator_id_start <= mse.id AND mse.id < mlm.operator_id_end)
FROM mz_introspection.mz_lir_mapping mlm
CROSS JOIN generate_series((mlm.operator_id_start) :: int8, (mlm.operator_id_end - 1) :: int8) AS valid_id
JOIN mz_introspection.mz_scheduling_elapsed_per_worker mse
ON (mse.id = valid_id)
GROUP BY mlm.global_id, mlm.lir_id"#,
));
from.push("LEFT JOIN summary_cpu sc USING (global_id, lir_id)");
Expand All @@ -922,9 +925,10 @@ GROUP BY mlm.global_id, mlm.lir_id"#,
mlm.lir_id AS lir_id,
mse.worker_id AS worker_id,
SUM(mse.elapsed_ns) AS worker_ns
FROM mz_introspection.mz_lir_mapping mlm
JOIN mz_introspection.mz_scheduling_elapsed_per_worker mse
ON (mlm.operator_id_start <= mse.id AND mse.id < mlm.operator_id_end)
FROM mz_introspection.mz_lir_mapping mlm
CROSS JOIN generate_series((mlm.operator_id_start) :: int8, (mlm.operator_id_end - 1) :: int8) AS valid_id
JOIN mz_introspection.mz_scheduling_elapsed_per_worker mse
ON (mse.id = valid_id)
GROUP BY mlm.global_id, mlm.lir_id, mse.worker_id"#,
));
from.push("LEFT JOIN per_worker_cpu pwc USING (global_id, lir_id)");
Expand Down Expand Up @@ -955,10 +959,11 @@ GROUP BY mlm.global_id, mlm.lir_id, mse.worker_id"#,
"megsa.levels AS levels",
"megsa.to_cut AS to_cut",
"megsa.hint AS hint",
"pg_size_pretty(savings) AS savings",
"pg_size_pretty(megsa.savings) AS savings",
]);
from.extend(["JOIN mz_introspection.mz_dataflow_global_ids mdgi ON (mlm.global_id = mdgi.global_id)",
"LEFT JOIN mz_introspection.mz_expected_group_size_advice megsa ON (megsa.dataflow_id = mdgi.id AND mlm.operator_id_start <= megsa.region_id AND megsa.region_id < mlm.operator_id_end)"]);
"LEFT JOIN (generate_series((mlm.operator_id_start) :: int8, (mlm.operator_id_end - 1) :: int8) AS valid_id JOIN \
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@ggevay ggevay Sep 18, 2025 •
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I'm wondering if this generate_series could be on the left side of the left join. Having it on the right side means that the right side is correlated with the left side, which is a complicated left join lowering case, with its own code path.

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I started with it on the left side, but I got redundant rows. (There may be a good way to hide those, but my SQL skill wasn't sufficient to the task.) The resulting plan is not small, but there is no cross join...

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I'm going to merge after a tiny doc update, but I made https://github.com/MaterializeInc/database-issues/issues/9730 to record your idea.

mz_introspection.mz_expected_group_size_advice megsa ON (megsa.region_id = valid_id)) ON (megsa.dataflow_id = mdgi.id)"]);
}
}

Expand Down
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