HashAggregate spill to disk and hash_mem_multiplier (work_mem * multiplier ceiling)
Simple terms
A GROUP BY builds a hash table with one entry per group and holds it in memory. Memory has a budget. Outgrow it and Postgres doesn't fail, and it doesn't degrade gently, it starts writing the newer groups out to temporary files and finishing them in later passes. EXPLAIN tells you when that happened: extra batches, and disk usage on the HashAggregate node. The answer is still correct. It just did a great deal more I/O to get there.
You might be asked
How does HashAggregate decide it has run out of memory, and what is hash_mem_multiplier? Walk me through what 'Batches' and 'Disk Usage' mean in an EXPLAIN ANALYZE HashAggregate node, and why a GROUP BY that was fine in PostgreSQL 11 can behave very differently in 13+.
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