From: Tom Lane <tgl@sss.pgh.pa.us>
To: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
cc: Maciek Sakrejda <m.sakrejda@gmail.com>, pgsql-docs@postgresql.org
Subject: Re: ORDER BY in materialized view example?
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Comments: In-reply-to Peter Eisentraut <peter.eisentraut@enterprisedb.com>
	message dated "Tue, 23 Nov 2021 17:06:32 +0100"
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Date: Tue, 23 Nov 2021 12:44:56 -0500
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Peter Eisentraut <peter.eisentraut@enterprisedb.com> writes:
> On 23.11.21 07:18, Maciek Sakrejda wrote:
>> An example in the materialized view documentation [1] includes an ORDER 
>> BY clause without a clear reason. Does it help build the index more 
>> efficiently? I suppose it's also sort of like a CLUSTER?

> I agree the ORDER BY is not relevant to the example.  There might be 
> some implementation-dependent advantage to ordering a materialized view, 
> but if there is, it isn't explained in the example.

Yeah.  It would result in the initial contents of the matview being
ordered, but I'm sure we don't wish to guarantee that REFRESH would
preserve that.  I'm on board with just removing the ORDER BY from
that example.

There is also this rather opaque "note" in the REFRESH MATERIALIZED VIEW
man page:

    While the default index for future CLUSTER operations is retained,
    REFRESH MATERIALIZED VIEW does not order the generated rows based on
    this property. If you want the data to be ordered upon generation, you
    must use an ORDER BY clause in the backing query.

I'd rather say something like

    If there is an ORDER BY clause in the matview's defining query,
    the original contents of the matview will be ordered that way;
    but REFRESH MATERIALIZED VIEW does not guarantee to preserve
    that ordering.

			regards, tom lane