Add domain to TODO.detail
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Bruce Momjian
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From pgsql-hackers-owner+M8916@postgresql.org Thu May 17 11:51:45 2001
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for <pgsql-hackers@postgreSQL.org>; Thu, 17 May 2001 11:22:21 -0400 (EDT)
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for <pgsql-hackers@postgreSQL.org>; Thu, 17 May 2001 17:22:16 +0200
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Subject: [HACKERS] possible DOMAIN implementation
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X-MimeOLE: Produced By Microsoft Exchange V6.0.4418.65
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Date: Thu, 17 May 2001 17:22:05 +0200
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Message-ID: <9C8918CC8CDAAC4AA79D4594A480648901E68C2B@EXCHANGE.mmp.plzen-city.cz>
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Thread-Topic: possible DOMAIN implementation
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Thread-Index: AcDe5WK0+5JgskVMEdWdCQAQS2oI5Q==
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From: =?iso-8859-2?Q?Hor=E1k_Daniel?= <horak@sit.plzen-city.cz>
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To: <pgsql-hackers@postgresql.org>
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Sender: pgsql-hackers-owner@postgresql.org
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Status: OR
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Hello,
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I have spend some thinking about implementation of DOMAIN capability.
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Here are my ideas.
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What is a domain? It is an alias for a type with size, constraints and
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default values. It is like one column of a table. And this is the main
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idea of my "implementation". It should be possible to implement it using
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existing system tables.
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New rules for grammar can be easily created from already existing pieces
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(column definition of a table).
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How to store information about a domain in system tables?
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When a new domain is created it will:
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- put a record into pg_type with typnam = domain name, new code for
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typtype = 'd' and typrelid = oid of a new record in pg_class (next line)
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- put a record into pg_class to create a fictional table with a new
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relkind ('d'?), relnatts = 1, relname can be system generated
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(pg_d_<domainname>)
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- put a records into pg_attribute and pg_attrdef with "column
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(attribute) definition" - real type, size, default value etc., owner
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will the fictional table from the previous step
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Then it will be required to modify functions that works with types. When
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typtype of a retrieved type is 'd' then it will perform lookups into
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pg_class, pg_attribute and pg_attrdef to find the real definition of the
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domain. These additional lookups will also create a performace penalty
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of using domains. But every feature has its costs. I know this paragraph
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about the real implementation is very short, but I think there are
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people that know the "type mechanism" better then I know. And can easier
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tell if it is possible to go this way.
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I hope you understand my explanation. It is also possible that I don't
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know some aspects of the backend code that makes my idea wrong.
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Dan
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----------------------------------------------
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Ing. Daniel Horak
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network and system administrator
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e-mail: horak@sit.plzen-city.cz
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privat e-mail: dan.horak@email.cz ICQ:36448176
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----------------------------------------------
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From pgsql-hackers-owner+M8925@postgresql.org Thu May 17 14:04:29 2001
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Thu, 17 May 2001 13:58:45 -0400 (EDT)
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To: =?iso-8859-2?Q?Hor=E1k_Daniel?= <horak@sit.plzen-city.cz>
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cc: pgsql-hackers@postgresql.org
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Subject: Re: [HACKERS] possible DOMAIN implementation
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In-Reply-To: <9C8918CC8CDAAC4AA79D4594A480648901E68C2B@EXCHANGE.mmp.plzen-city.cz>
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References: <9C8918CC8CDAAC4AA79D4594A480648901E68C2B@EXCHANGE.mmp.plzen-city.cz>
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Comments: In-reply-to =?iso-8859-2?Q?Hor=E1k_Daniel?= <horak@sit.plzen-city.cz>
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message dated "Thu, 17 May 2001 17:22:05 +0200"
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Date: Thu, 17 May 2001 13:58:45 -0400
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Message-ID: <12167.990122325@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Precedence: bulk
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Sender: pgsql-hackers-owner@postgresql.org
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Status: OR
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=?iso-8859-2?Q?Hor=E1k_Daniel?= <horak@sit.plzen-city.cz> writes:
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> When a new domain is created it will:
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> - put a record into pg_type with typnam = domain name, new code for
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> typtype = 'd' and typrelid = oid of a new record in pg_class (next line)
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> - put a record into pg_class to create a fictional table with a new
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> relkind ('d'?), relnatts = 1, relname can be system generated
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> (pg_d_<domainname>)
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Ugh. Don't overload pg_class with things that are not tables. I see no
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reason that either pg_class or pg_attribute should be involved in the
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definition of a domain. Make new system tables if you need to, but
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don't confuse the semantics of critical tables.
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> - put a records into pg_attribute and pg_attrdef with "column
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> (attribute) definition" - real type, size, default value etc., owner
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> will the fictional table from the previous step
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> Then it will be required to modify functions that works with types. When
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> typtype of a retrieved type is 'd' then it will perform lookups into
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> pg_class, pg_attribute and pg_attrdef to find the real definition of the
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> domain. These additional lookups will also create a performace penalty
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> of using domains.
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Why shouldn't this info be directly available from the pg_type row?
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regards, tom lane
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