Date: Mon, 2 Oct 2017 10:23:41 -0500
From: Nico Williams <nico@cryptonector.com>
To: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: generated columns
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References: <b151f851-4019-bdb1-699e-ebab07d2f40a@2ndquadrant.com>
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On Thu, Aug 31, 2017 at 12:16:43AM -0400, Peter Eisentraut wrote:
> In previous discussions, it has often been a source of confusion whether
> these generated columns are supposed to be computed on insert/update and
> stored, or computed when read.  The SQL standard is not explicit, but
> appears to lean toward stored.  DB2 stores.  Oracle computes on read.

Question: How would one know the difference between storing computed
          columns vs. computing them on read?

Answer?:  Performance.  If the computation is slow, then you'll really
          notice on read.

Nico
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