Relations summary at end of vacuum

[Doug Y]

Hi,
What exactly are the "Relations" that are referred to in the summary
at the end of a vacuum? I have two DBs that are supposed to be mirrored
copies of each other, but my relation & page counts are different for
each after a vacuum:

DB_a:
INFO: free space map: 109 relations, 70413 pages stored; 71696 total
pages needed
DETAIL: Allocated FSM size: 1000 relations + 100000 pages = 647 kB
shared memory.

DB_b:
INFO: free space map: 281 relations, 57954 pages stored; 61904 total
pages needed
DETAIL: Allocated FSM size: 1000 relations + 100000 pages = 647 kB
shared memory.

The schemas and DBs are set up exactly the same, so I'm rather confused
as to the discrepancy. Data checks (sums on key columns, etc) show that
the data does appear to be in sync. I checked pg_tables, pg_indexes, and
both have the same info.

If this is normal behavior, I'd just like some additional info to set my
mind at ease.

Thanks

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[Tom Lane]

Post by Doug Y
What exactly are the "Relations" that are referred to in the summary
at the end of a vacuum?

Tables and indexes.

Post by Doug Y
I have two DBs that are supposed to be mirrored copies of each other,

IIRC, a relation won't get entered into the free space map at all if
no useful free space is found by VACUUM. So if the only thing that ever
happens on the mirror is insertion of new rows, I could believe that
there would be many fewer active FSM entries than for the master.

However the fact that the database with fewer FSM relations knows about
more free pages within those tables seems a bit at variance with that
idea. Maybe your mirror is insert-only for some tables and not others,
and you've been lax about vacuuming the mirror?

regards, tom lane

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