Message-ID: <555E0488.1030908@boscorama.com>
Date: Thu, 21 May 2015 09:15:04 -0700
From: Bosco Rama <postgres@boscorama.com>
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To: "David G. Johnston" <david.g.johnston@gmail.com>,
 "er.tejaspatel88@gmail.com" <er.tejaspatel88@gmail.com>
CC: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Fastest way / best practice to calculate "next birthdays"
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On 05/20/15 20:22, David G. Johnston wrote:
> On Monday, May 18, 2015, er.tejaspatel88@gmail.com <
> er.tejaspatel88@gmail.com> wrote:
> 
>> If I have to find upcoming birthdays in current week and the current week
>> fall into different months - how would you handle that?
>>
> 
> Extract(week from timestamptz_column)
> 
> ISO weeks are not affected by month boundaries but do start on Monday.

There is the year start/end boundary conditions to worry about there.

If the current week covers Dec28-Jan02 then week of year won't help for
a birthday on Jan01 or Jan02 if 'today' is in the Dec portion.  Ditto
for birthday in Dec portion when 'today' is in the Jan portion.

There is probably a better way to do it than what I'm showing here, but
here's an example:

with x as (
   select now() - (extract(dow from now()) || ' days')::interval as
weekstart
)
select to_char(x.weekstart, 'YYYY-MM-DD') as first_day,
        to_char(x.weekstart + '6 days', 'YYYY-MM-DD') as last_day
    from x;

You could probably make some of that into a function that accepts a
timestamptz and generates the two days.  Or even does the compare too.

HTH.

Bosco.


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