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From: Dean Gibson (DB Administrator) <postgresql@mailpen.com>
To: pgsql-performance@postgresql.org
Subject: Re: AWS forcing PG upgrade from v9.6 a disaster
Date: Sun, 6 Jun 2021 16:49:24 -0700
Message-ID: <79489b9a-608e-4a96-076a-963e7217fc3c@mailpen.com> (raw)
In-Reply-To: <5d3aa25f-a886-a3e8-ba1c-9d31c30a21c3@dunslane.net>
References: <3fa6fd38-1f92-98ab-0055-3e0eb7e4e772@ultimeth.com>
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	<70edb767-6ba0-1065-2f74-f01440e3caea@gmail.com>
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	<2b7b9a25-e3c7-b980-a1e3-d21d8d91d34d@gmail.com>
	<988e2438-c322-c1d5-ca88-683943a80087@mailpen.com>
	<1a2f01ce-b3da-3066-0642-3e311587ece3@aklaver.com>
	<94ed2da7-7622-e57b-1562-3f7caf40b4a8@mailpen.com>
	<5d3aa25f-a886-a3e8-ba1c-9d31c30a21c3@dunslane.net>

On 2021-05-29 13:35, Andrew Dunstan wrote:
> On 5/29/21 3:59 PM, Dean Gibson (DB Administrator) wrote:
>> Meanwhile, I've been doing some checking.  If I remove "CAST(
>> license_status AS CHAR ) = 'A'", the problem disappears.  Changing the
>> JOIN to a RIGHT JOIN, & replacing WHERE with ON, also "solves" the
>> problem, but there is an extra row where license_status is NULL, due
>> to the RIGHT JOIN.  Currently trying to figure that out (why did the
>> CAST ... match 'A', if it is null?)...
> Why are you using this expression? It's something you almost never want
> to do in my experience. Why not use the substr() function to get the
> first character?
>
> cheers
>
> andrew
>
> --
> Andrew Dunstan
> EDB: https://www.enterprisedb.com

Although it doesn't matter in this case, I do it because in general, it 
changes the type of the value from CHAR to bptext or whatever it is, & 
that has causes comparison issues in the past.  It's just a matter of 
habit for me when working with CHAR() types.

But this case, where it doesn't matter, I'd use LEFT().

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