YoannAbriel opened a new pull request, #63797:
URL: https://github.com/apache/airflow/pull/63797
Migration 0101 paginates over `serialized_dag` using `ORDER BY dag_id`, but
`dag_id` has no index. On MySQL with large tables (100K+ rows), the inner
subquery's filesort exceeds `sort_buffer_size` and fails with
`OperationalError: (1038, 'Out of sort memory')`.
The existing subquery optimization (selecting only `id, dag_id` in the inner
query) isn't enough — without an index, MySQL still has to sort the full result
set in memory.
Fix: create a temporary index on `serialized_dag(dag_id)` before the
pagination loop and drop it after, in both upgrade and downgrade paths. The
index lets MySQL use an index scan instead of a filesort, eliminating the sort
buffer dependency entirely.
Closes: #63786
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##### Was generative AI tooling used to co-author this PR?
- [X] Yes — Claude Code (Opus 4, claude-opus-4-6)
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