Message-ID: <43CADF2C.7010100@ultimeth.com>
Date: Sun, 15 Jan 2006 15:47:56 -0800
From: "Dean Gibson (DB Administrator)" <postgresql4@ultimeth.com>
Organization: UltiMeth Systems
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To: pgsql-sql <pgsql-sql@postgresql.org>
Subject: Re: What does "merge-joinable join conditions" mean ????
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On 2006-01-15 15:42, Tom Lane wrote:
>   
>> EXPLAIN SELECT count(*)  FROM "Extra" FULL JOIN  "GeoRestrict" ON 
>> callsign ~ pattern  WHERE geo_region = 
>> 4;                                            
>>     
>
> Oh, but that reduces it to a left join, as you can see in the EXPLAIN
> output ---
Yes, I previously noticed that in the EXPLAIN output too.

>  any null-extension rows from the right side are going to fail
> the WHERE condition anyway, so the planner simplifies the FULL JOIN to a
> LEFT JOIN.  If you'd eliminate the WHERE altogether then the failure
> will come back.
>
>   
Tried that, and you are right there as well.

So, given the fact that the right-hand-table is only about 15 rows, do 
you think changing the SELECT back to a LEFT JOIN, and then using a 
UNION to get the extra right-hand-rows in, is the best work-around?

-- Dean