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Re: Check constraints on partition parents only?

From: Andrew Dunstan <andrew(at)dunslane(dot)net>
To: Jerry Sievers <gsievers19(at)comcast(dot)net>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Check constraints on partition parents only?
Date: 2011-07-26 02:44:32
Message-ID: 4E2E2A10.8000904@dunslane.net (view raw or flat)
Thread:
Lists: pgsql-hackers

On 07/25/2011 10:31 PM, Jerry Sievers wrote:
> Hackers;
>
> I just noticed that somewhere between 8.2 and 8.4, an exception is
> raised trying to alter table ONLY some_partition_parent ADD CHECK
> (foo).
>
> I can understand why it makes sense to handle this as an error.
>
> Howeverin practice on a few systems that I used to manage this would
> be a problem.
>
> 1. I got into the habit of putting CHECK (false) on the parent table
>     if it was an always empty base table,
>
>     This is just really documentation indicating that this table can't
>     hold rows and of course, having the partition selector trigger
>     raise exception if falling through the if/else logic on a new row
>     insertion enforces the constraint but is less obvious.
>
>     Ok, so no real problem here.  Just one example.
>
> 2. Atypical partitioning implementation where the parent table was for
>     initial insert/update of "live" records in an OLTP system with high
>     update/insert ratio.  This table was partitioned retroactively in
>     such a way transparent to the application.  The app would
>     eventually update a row one final time and set a status field to
>     some terminal status, at which time we'd fire a trigger to move the
>     row down into a partition.  Record expiry took place periodically
>     by dropping a partition and creating a new one.
>
>     In that case, imagine the application user runs with
>     sql_inheritance to off and so, sees only the live data which
>     resulted in a huge performance boost.  Reporting apps and in fact
>     all other users ran with sql_inheritance to on as usual and so, see
>     all the data.
>
>     Suppose the status field had several non-terminal values and one or
>     a few terminal values.  The differing check constraints on parent
>     and child tables made it easy to see the intent and I presume with
>     constraint_exclusion set to on, let queries on behalf of regular
>     users that had specified a non-terminal state visit only the tiny
>     parent table.
>
>     Parent might have CHECK (status in (1,2,3)) and children CHECK
>     (status = 4).
>
>     I'll assume not many sites are architected this way but #2 here
>     shows a more compelling example of why it might be useful to allow
>     check constraints added to only a partition parent.


8.4 had this change:

        *

          Force child tables to inherit CHECK constraints from parents
          (Alex Hunsaker, Nikhil Sontakke, Tom)

          Formerly it was possible to drop such a constraint from a
          child table, allowing rows that violate the constraint to be
          visible when scanning the parent table. This was deemed
          inconsistent, as well as contrary to SQL standard.


You're not the only one who occasionally bangs his head against it.

cheers

andrew





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