Message-ID: <42F015E3.8070308@travelamericas.com>
Date: Tue, 02 Aug 2005 17:54:59 -0700
From: Chris Travers <chris@travelamericas.com>
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To: Kyle Bateman <kyle@actarg.com>,
	pgsql-sql <pgsql-sql@postgresql.org>
Subject: Re: Trigger on select?
References: <42F01AAF.6040903@actarg.com>
In-Reply-To: <42F01AAF.6040903@actarg.com>
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Kyle Bateman wrote:

> Hey, anyone know if it is possible to fire a trigger before a select?  
> I'm considering creating some tables which contain data summarized 
> from other tables as kind of a cache mechanism.  The hope is I can 
> speed up some queries that get executed a lot (and are kind of slow) 
> by maintaining data (like sums of big columns, for example).  I was 
> all ready to go and then I discovered that trigger-before-select is 
> not supported.  (Is it in any DB?)
>
> The idea is this:
>
> Any time I execute a query that would access the summary data, the 
> "before select" trigger fires and goes out and builds any summary data 
> missing from the summary table.

No.  You must instead generate a view.

> When I do an insert,update,delete on the primary data table, another 
> trigger fires that removes the applicable data from the summary 
> table.  This way, I only cache the information I need in the summary 
> table, right before I need it.  But it can stay there as long as the 
> base information doesn't change so I don't have to redo the expensive 
> operation of summarizing it any more often than necessary.  Its kind 
> of like an index in a way, but it is not maintained at insert/update 
> time.  Rather, it is updated as it is needed.
>
> Anyone have any ideas about how I can accomplish this?


something like create view wrapper_table as
select * from original table where (select pseudo_trigger_function())  
IS TRUE;

The above example is off the top of my head.  It may require some editing.

Best Wishes,
Chris Travers
Metatron Technology Consulting