From: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Subject: Re: Surjective functional indexes
To: Konstantin Knizhnik <k.knizhnik@postgrespro.ru>,
 PostgreSQL Hackers <pgsql-hackers@postgresql.org>
References: <4d9928ee-a9e6-15f9-9c82-5981f13ffca6@postgrespro.ru>
Organization: 2ndQuadrant
Message-ID: <b9929458-ba99-1ccd-ea61-181b852f9ec1@2ndquadrant.com>
Date: Sat, 27 May 2017 14:50:03 -0400
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On 5/25/17 12:30, Konstantin Knizhnik wrote:
> Functions like (info->>'name') are named "surjective" ni mathematics.

A surjective function is one where each value in the output type can be
obtained by some input value.  That's not what you are after here.  The
behavior you are describing is a not-injective function.

I think you are right that in practice most functions are not injective.
 But I think there is still quite some difference between a function
like the one you showed that selects a component from a composite data
structure and, for example, round(), where in practice any update is
likely to change the result of the function.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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