Date: Fri, 19 Jul 2024 16:21:37 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: Daniel Gustafsson <daniel@yesql.se>
Cc: Robert Haas <robertmhaas@gmail.com>, Jeff Davis <pgsql@j-davis.com>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: optimizing pg_upgrade's once-in-each-database steps
Message-ID: <ZprY4e23vR95Nc51@nathan>
References: <3b97ab8e295ec3db22baa2dfdd49073449ae1696.camel@j-davis.com>
 <20240517012408.GA1691322@nathanxps13>
 <ZoMHsBasBy8LqkEU@nathan>
 <CA+TgmoaBof36-ZuRWtu=+8rnhGTNQFFGNcmVB0x3aQawfc_JxA@mail.gmail.com>
 <ZoMK9ZhOsWEKqxlM@nathan>
 <ZowEIGd0lLKh16qe@nathan>
 <ZoyveaS6a6jyhYS6@nathan>
 <26742670-9539-4F30-B6FA-9C1DB743BC23@yesql.se>
 <Zpg4WnS9BRWbgBLi@nathan>
 <4493D323-9726-48F7-A702-E71FB991F76B@yesql.se>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <4493D323-9726-48F7-A702-E71FB991F76B@yesql.se>
Archived-At: <https://www.postgresql.org/message-id/ZprY4e23vR95Nc51%40nathan>
Precedence: bulk

On Thu, Jul 18, 2024 at 09:57:23AM +0200, Daniel Gustafsson wrote:
>> On 17 Jul 2024, at 23:32, Nathan Bossart <nathandbossart@gmail.com> wrote:
>> On Wed, Jul 17, 2024 at 11:16:59PM +0200, Daniel Gustafsson wrote:
> 
>>> +static void
>>> +dispatch_query(const ClusterInfo *cluster, AsyncSlot *slot,
>>>    ....
>>> +   pg_free(query);
>>> +}
>>> 
>>> A minor point, perhaps fueled by me not having played around much with this
>>> patchset.  It seems a bit odd that dispatch_query is responsible for freeing
>>> the query from the get_query callback.  I would have expected the output from
>>> AsyncTaskGetQueryCB to be stored in AsyncTask and released by async_task_free.
>> 
>> I don't see any problem with doing it the way you suggest.

Actually, I do see a problem.  If we do it this way, we'll have to store a
string per database somewhere, which seems unnecessary.

However, while looking into this, I noticed that only one get_query
callback (get_db_subscription_count()) actually customizes the generated
query using information in the provided DbInfo.  AFAICT we can do this
particular step without running a query in each database, as I mentioned
elsewhere [0].  That should speed things up a bit and allow us to simplify
the AsyncTask code.

With that, if we are willing to assume that a given get_query callback will
generate the same string for all databases (and I think we should), we can
run the callback once and save the string in the step for dispatch_query()
to use.  This would look more like what you suggested in the quoted text.

[0] https://postgr.es/m/ZprQJv_TxccN3tkr%40nathan

-- 
nathan