Subject: Re: Materialized View Estimation in Postgres
To: pgsql-admin@lists.postgresql.org
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From: Thomas Kellerer <shammat@gmx.net>
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Date: Wed, 15 Apr 2020 22:52:35 +0200
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> I would like to know if there are any oracle equivalent of
> dbms_mview.estimate_mview_size in Postgres as we have been using it
> in Oracle for estimating required size for the upcoming mviews but
> after migration to Postgres we find it very hard to estimate the
> size.. Sometime our rds ran out of space and we had to spend lot of
> efforts and time to bring back the rds once after rds storage is
> exhausted. Or is there any other options available which can be used
> to estimate the size.

A very rough estimation could be to use pg_column_size() on the rows of your SELECT statement and aggregate that.
Despite it's name pg_column_size() also calculates the size of a complete row.

   select sum(pg_column_size(mv))
   from (
     .... your select statement here ...
   ) mv;