Message-ID: <51CC8F1A.1090805@optionshouse.com>
Date: Thu, 27 Jun 2013 14:14:34 -0500
From: Shaun Thomas <sthomas@optionshouse.com>
Reply-To: <sthomas@optionshouse.com>
Organization: Peak6
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To: Tom Lane <tgl@sss.pgh.pa.us>
CC: Dave Johansen <davejohansen@gmail.com>, bricklen <bricklen@gmail.com>,
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Subject: Re: Partitions not Working as Expected
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On 06/27/2013 01:42 PM, Tom Lane wrote:

> That will break things: CURRENT_DATE will then be equivalent to just
> writing today's date as a literal.

Interesting. I tested it by creating a view and a table with a default, 
and it always seems to get translated to:

('now'::text)::date

But I'll take your explanation at face value, since that doesn't imply 
what the output would be. What's interesting is that EnterpriseDB has 
their own pg_catalog.current_date function that gets called by the 
CURRENT_DATE keyword. So unlike in vanilla PG, I could mark just the 
current_date function as immutable without affecting a lot of other 
internals.

On EDB, this actually works:

UPDATE pg_proc
    SET provolatile = 'i'
  WHERE proname = 'current_date';

Then the plan gets pared down as desired. But again, if the date were to 
roll over, I'm not sure what would happen. I wish I could test that 
without fiddling with machine times.

> I don't see any very good solution to your problem within the current
> approach to partitioning, which is basically theorem-proving.  That
> proof engine has no concept of time passing, let alone the sort of
> detailed knowledge of the semantics of this particular function that
> would allow it to conclude "if CURRENT_DATE > '2013-06-20' is true now,
> it will always be so in the future as well".

I get it. From the context of two months ago, CURRENT_DATE > 
'2013-06-20' would return a different answer than it would today, which 
isn't really good for proofs.

The only way for it to work as "expected" would be to add a first pass 
to resolve any immediate variables, which would effectively throw away 
plan caches. I'd actually be OK with that.

> I think most hackers agree that the way forward on partitioning
> involves building hard-wired logic that selects the correct
> partition(s) at run-time, so that it wouldn't particularly matter
> where we got the comparison value from or whether it was a constant.

Fair enough. I'll stop telling devs to use current_date instead of ORM 
injections, then. Hopefully we can track down and tweak the affected 
queries on the tables we're partitioning without too much work and QA.

Thanks, Tom!

-- 
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-676-8870
sthomas@optionshouse.com

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