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From: Greg Stark <[email protected]>
To: Andres Freund <[email protected]>
Cc: Kyotaro Horiguchi <[email protected]>
Cc: Melanie Plageman <[email protected]>
Cc: Justin Pryzby <[email protected]>
Cc: Thomas Munro <[email protected]>
Cc: David G. Johnston <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Subject: Re: shared-memory based stats collector - v70
Date: Wed, 20 Jul 2022 11:35:13 -0400
Message-ID: <CAM-w4HN=4ss5dS2DH6Cof=zocJXeGOjRNKxEGgUZdJPMndyNzA@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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So I'm finally wrapping my head around this new code. There is
something I'm surprised by that perhaps I'm misreading or perhaps I
shouldn't be surprised by, not sure.

Is it true that the shared memory allocation contains the hash table
entry and body of every object in every database? I guess I was
assuming I would find some kind of LRU cache which loaded data from
disk on demand. But afaict it loads everything on startup and then
never loads from disk later. The disk is purely for recovering state
after a restart.

On the one hand the rest of Postgres seems to be designed on the
assumption that the number of tables and database objects is limited
only by disk space. The catalogs are stored in relational storage
which is read through the buffer cache. On the other hand it's true
that syscaches don't do expire entries (though I think the assumption
is that no one backend touches very much).

It seems like if we really think the total number of database objects
is reasonably limited to scales that fit in RAM there would be a much
simpler database design that would just store the catalog tables in
simple in-memory data structures and map them all on startup without
doing all the work Postgres does to make relational storage scale.

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