fix(optimizer)!: merge window issue

[fivetran-kwoodbeck]

Fixes a correctness bug in merge_subqueries.py where a window function subquery was incorrectly merged into an outer query that filters or joins on its input rows. This alters the query to where it would produce different / wrong results.

Problem query:

SELECT s.w FROM (SELECT a, ROW_NUMBER() OVER (ORDER BY a) AS w FROM x) AS s WHERE s.a > 5

Before fix:

SELECT ROW_NUMBER() OVER (ORDER BY `a`) AS `w` FROM `_t0` AS `_t0` WHERE `a` > 5

After fix:

SELECT `_t1`.`_c1` AS `w` FROM (SELECT `_c0` AS `_c0`, ROW_NUMBER() OVER (ORDER BY `_c0`) AS `_c1` FROM `_t0` AS `_t0`) AS `_t1` WHERE `_t1`.`_c0` > 5

/integration-tests

[github-actions]

SQLGlot Integration Test Results

✅ All tests passed

Comparing:

• this branch (sqlglot:optimizer/bug-merge-window @ sqlglot a99aad7)
• baseline (main @ sqlglot 77c7814)

Overall

main: 192416 total, 153532 passed (pass rate: 79.8%)

sqlglot:optimizer/bug-merge-window: 192416 total, 153532 passed (pass rate: 79.8%)

Transitions:
No change

✅ All tests passed

[georgesittas]

Nice find!