Aggregate

memory

{"description":"This implementation contains a disk-spill path for hashed aggregation. Aggregate input sorting and transition-state allocations have their own behavior; one spill policy does not describe every aggregation strategy.","evidence":[{"archive_sha256":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","label":"src/backend/executor/nodeAgg.c","path":"src/backend/executor/nodeAgg.c","sha256":"8719569a73f054e9026c0a00c814f60b6c7879753c58e4e6dcc38fee333c45c5","url":"https://ftp.postgresql.org/pub/source/v18.6/postgresql-18.6.tar.bz2"}],"mechanism":"aggregate-spill","source_notes":["When performing hash aggregation, if the hash table memory exceeds the limit (see hash_agg_check_limits()), we enter \"spill mode\". In spill mode, we advance the transition states only for groups already in the hash table. For tuples that would need to create a new hash table entries (and initialize new transition states), we instead spill them to disk to be processed later. The tuples are spilled in a partitioned manner, so that subsequent batches are smaller and less likely to exceed hash_mem (if a batch does exceed hash_mem, it must be spilled recursively).","Spilled data is written to logical tapes. These provide better control over memory usage, disk space, and the number of files than if we were to use a BufFile for each spill. We don't know the number of tapes needed at the start of the algorithm (because it can recurse), so a tape set is allocated at the beginning, and individual tapes are created as needed. As a particular tape is read, logtape.c recycles its disk space. When a tape is read to completion, it is destroyed entirely.","Control how many partitions are created when spilling HashAgg to disk.","We also specify a min and max number of partitions per spill. Too few might mean a lot of wasted I/O from repeated spilling of the same tuples. Too many will result in lots of memory wasted buffering the spill files (which could instead be spent on a larger hash table)."]}

source inventory

{"executor":"src/backend/executor/execProcnode.c","explain":"src/backend/commands/explain.c","implementation":"src/backend/executor/nodeAgg.c"}