maintenance_work_mem

The memory a vacuum may use for its dead-TID store, default 64 MB. Before Postgres 17 this was the hidden multiplier on vacuum cost: 64 MB held about 11M TIDs, and a table with more dead rows than that forced vacuum to stop, scan every index, and start again: multiple full index passes per run, capped further by a hard 1 GB limit. Postgres 17 replaced the array with an adaptive radix tree: ~20× more TIDs per MB, no 1 GB cap, and almost always a single index pass.