Re: How to hint 2 coulms IS NOT DISTINCT FROM each other

On 4 November 2016 at 14:41, Merlin Moncure <mmoncure(at)gmail(dot)com> wrote:
> On Fri, Nov 4, 2016 at 8:08 AM, Kim Rose Carlsen <krc(at)hiper(dot)dk> wrote:
>> The nulls are generated by something like this
>> SELECT c.circuit_id,
>> cc.customer_id
>> FROM circuit AS c
>> LEFT JOIN circuit_customer AS cc
>> ON c.circuit_id = cc.circuit_id
>>
>> To make a magic '0' customer we would be required to use
>> COALESCE(cc.customer_id, '0')
>> I dont think the optimizer will do anything clever with the '0' we have
>> computed from null.
>
> It would if you explicitly indexed it as such;
> CREATE INDEX ON circuit_customer((COALESCE(customer_id, '0'));

Merlin, it's a LEFT JOIN. There probably are no NULLs in the
circuit_customer.customer_id column, so that COALESCE isn't going to
achieve anything at all.

I haven't been following this particular discussion in detail, so
unfortunately I can't contribute more than that remark at the moment.

--
If you can't see the forest for the trees,
Cut the trees and you'll see there is no forest.