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fix(optimizer): convert INNER JOINs to LEFT JOINs when merging LEFT JOIN subqueries #5980

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Merged
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Sep 30, 2025
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fix(optimizer): convert INNER JOINs to LEFT JOINs when merging LEFT JOIN subqueries #5980

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fix(optimizer): convert INNER JOINs to LEFT JOINs when merging LEFT J…
geooo109 Sep 27, 2025
841d3bc
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geooo109 Sep 27, 2025
a15b6bd
refactor names
geooo109 Sep 27, 2025
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7 changes: 7 additions & 0 deletions sqlglot/optimizer/merge_subqueries.py
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Original file line number Diff line number Diff line change
Expand Up @@ -282,7 +282,14 @@ def _merge_joins(outer_scope: Scope, inner_scope: Scope, from_or_join: FromOrJoi
new_joins = []

joins = inner_scope.expression.args.get("joins") or []

outer_is_left_join = isinstance(from_or_join, exp.Join) and from_or_join.side == "LEFT"

for join in joins:
if outer_is_left_join and not join.method and join.kind in ("", "INNER"):
join.args.pop("kind", None)
join.set("side", "LEFT")

new_joins.append(join)
outer_scope.add_source(join.alias_or_name, inner_scope.sources[join.alias_or_name])

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83 changes: 83 additions & 0 deletions tests/fixtures/optimizer/merge_subqueries.sql
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Expand Up @@ -511,3 +511,86 @@ with cte as (
)
select cte.mult from cte;
SELECT x.a * x.b AS mult FROM x AS x;

# title: replace INNER JOIN with LEFT JOIN when it exists in a subquery used as the RHS of a LEFT JOIN
WITH
t0 AS (
SELECT 5 as id
),
t1 AS (
SELECT 1 AS id, 'US' AS cid
),
t2 AS (
SELECT 1 AS id, 'US' AS cid
)
SELECT
t0.id,
t3.cid AS cid
FROM t0
LEFT JOIN (
SELECT
t1.id,
t2.cid
FROM t1
INNER JOIN t2
ON t1.cid = t2.cid
) AS t3
ON t0.id = t3.id;
WITH t0 AS (SELECT 5 AS id), t1 AS (SELECT 1 AS id, 'US' AS cid), t2 AS (SELECT 1 AS id, 'US' AS cid) SELECT t0.id AS id, t2.cid AS cid FROM t0 AS t0 LEFT JOIN t1 AS t1 ON t0.id = t1.id LEFT JOIN t2 AS t2 ON t1.cid = t2.cid;

# title: preserve INNER JOIN when it exists in a subquery used as the RHS of a RIGHT JOIN
WITH
t0 AS (
SELECT 5 as id
),
t1 AS (
SELECT 1 AS id, 'US' AS cid
),
t2 AS (
SELECT 1 AS id, 'US' AS cid
)
SELECT
t0.id,
t3.cid AS cid
FROM t0
RIGHT JOIN (
SELECT
t1.id,
t2.cid
FROM t1
INNER JOIN t2
ON t1.cid = t2.cid
) AS t3
ON t0.id = t3.id;
WITH t0 AS (SELECT 5 AS id), t1 AS (SELECT 1 AS id, 'US' AS cid), t2 AS (SELECT 1 AS id, 'US' AS cid) SELECT t0.id AS id, t2.cid AS cid FROM t0 AS t0 RIGHT JOIN t1 AS t1 ON t0.id = t1.id INNER JOIN t2 AS t2 ON t1.cid = t2.cid;

# title: replace multiple INNER JOINS with LEFT JOINS when they exist in a subquery used as the RHS of a LEFT JOIN
WITH
t0 AS (
SELECT 5 as id
),
t1 AS (
SELECT 1 AS id, 'US' AS cid
),
t2 AS (
SELECT 1 AS id, 'US' AS cid
),
t3 AS (
SELECT 1 AS id, 'CA' AS cid
)
SELECT
t0.id,
t4.cid AS cid
FROM t0
LEFT JOIN (
SELECT
t1.id,
t3.cid
FROM t1
INNER JOIN t2
ON t1.cid = t2.cid
INNER JOIN t3
ON t2.id = t3.id
) AS t4
ON t0.id = t4.id;
WITH t0 AS (SELECT 5 AS id), t1 AS (SELECT 1 AS id, 'US' AS cid), t2 AS (SELECT 1 AS id, 'US' AS cid), t3 AS (SELECT 1 AS id, 'CA' AS cid) SELECT t0.id AS id, t3.cid AS cid FROM t0 AS t0 LEFT JOIN t1 AS t1 ON t0.id = t1.id LEFT JOIN t2 AS t2 ON t1.cid = t2.cid LEFT JOIN t3 AS t3 ON t2.id = t3.id;