Re: ALTER TABLE ADD COLUMN fast default

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andrew Dunstan <andrew(dot)dunstan(at)2ndquadrant(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: ALTER TABLE ADD COLUMN fast default
Date: 2017-12-06 15:16:34
Message-ID: 27349.1512573394@sss.pgh.pa.us
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Andrew Dunstan <andrew(dot)dunstan(at)2ndquadrant(dot)com> writes:
> Attached is a patch for $subject. It's based on Serge Reilau's patch of
> about a year ago, taking into account comments made at the time. with
> bitrot removed and other enhancements such as documentation.

> Essentially it stores a value in pg_attribute that is used when the
> stored tuple is missing the attribute. This works unless the default
> expression is volatile, in which case a table rewrite is forced as
> happens now.

I'm quite disturbed both by the sheer invasiveness of this patch
(eg, changing the API of heap_attisnull()) and by the amount of logic
it adds to fundamental tuple-deconstruction code paths. I think
we'll need to ask some hard questions about whether the feature gained
is worth the distributed performance loss that will ensue.

regards, tom lane

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