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How PostgreSQL sorts: quicksort, external merge, and top-N heapsort

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A sort tries to stay in memory, and work_mem is the budget it gets. Fits? One quicksort and you're done. Doesn't fit? Postgres sorts as much as it can hold, spills that run to a temporary file, repeats, then merges the runs back together, correct, but paying disk the whole way. There is a third case worth knowing too: ORDER BY with a LIMIT doesn't sort everything. It keeps a small heap of just the rows you asked for and discards the rest as it goes.

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EXPLAIN ANALYZE shows three different 'Sort Method' lines depending on the query: 'quicksort Memory', 'external merge Disk', and 'top-N heapsort Memory'. Walk me through what each one means, what makes PostgreSQL choose between them, and where work_mem comes into it.

TopicHow PostgreSQL sorts: quicksort vs external merge vs top-N heapsort, and the work_mem spill boundary
PostgreSQL17.10 (mechanism is the same on 14-17; method strings unchanged since 9.6)
Tools usedpgi17 lab (PostgreSQL 17.10), EXPLAIN (ANALYZE) Sort Method line, source @REL_17_10 src/backend/utils/sort/tuplesort.c
Last reviewed2026-06-19

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