maintenance_work_mem = 64kB doesn't work for vacuum

Hi,

Commit bbf668d66fbf6 (back-patched to v17) lowered the minimum
maintenance_work_mem to 64kB, but it doesn't work for parallel vacuum
cases since the minimum dsa segment size (DSA_MIN_SEGMENT_SIZE) is
256kB. As soon as the radix tree allocates its control object and the
root node, the memory usage exceeds the maintenance_work_mem limit and
vacuum ends up with the following assertion:

TRAP: failed Assert("vacrel->lpdead_item_pages > 0"), File:
"vacuumlazy.c", Line: 2447, PID: 3208575

On build without --enable-cassert, vacuum never ends.

I've tried to lower DSA_MIN_SEGMENT_SIZE to 64kB but no luck.
Investigating it further, dsa creates a 64kB superblock for each size
class and when creating a new shared radix tree we need to create two
superblocks: one for the radix tree control data (64 bytes) and
another one for the root node (40 bytes). Also, each superblock
requires a span data, which uses 1 page (4096kB). Therefore, we need
at least 136kB for a shared radix tree even when it's empty.

A simple fix is to bump the minimum maintenance_work_mem to 256kB. We
would break the compatibility for backbranch (i.e. v17) but I guess
it's unlikely that existing v17 users are using less than 1MB
maintenance_work_mem (the release note doesn't mention the fact that
we lowered the minimum value).

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com