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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 17:22:16 +0200
+Subject: [HACKERS] possible DOMAIN implementation
+MIME-Version: 1.0
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+ charset="iso-8859-2"
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+Date: Thu, 17 May 2001 17:22:05 +0200
+Message-ID: <9C8918CC8CDAAC4AA79D4594A480648901E68C2B@EXCHANGE.mmp.plzen-city.cz>
+Thread-Topic: possible DOMAIN implementation
+Thread-Index: AcDe5WK0+5JgskVMEdWdCQAQS2oI5Q==
+From: =?iso-8859-2?Q?Hor=E1k_Daniel?= <horak@sit.plzen-city.cz>
+To: <pgsql-hackers@postgresql.org>
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+Sender: pgsql-hackers-owner@postgresql.org
+Status: OR
+
+Hello,
+
+I have spend some thinking about implementation of DOMAIN capability.
+Here are my ideas.
+
+What is a domain? It is an alias for a type with size, constraints and
+default values. It is like one column of a table. And this is the main
+idea of my "implementation". It should be possible to implement it using
+existing system tables.
+
+New rules for grammar can be easily created from already existing pieces
+(column definition of a table).
+
+How to store information about a domain in system tables?
+When a new domain is created it will:
+- put a record into pg_type with typnam = domain name, new code for
+typtype = 'd' and typrelid = oid of a new record in pg_class (next line)
+- put a record into pg_class to create a fictional table with a new
+relkind ('d'?), relnatts = 1, relname can be system generated
+(pg_d_<domainname>)
+- put a records into pg_attribute and pg_attrdef with "column
+(attribute) definition" - real type, size, default value etc., owner
+will the fictional table from the previous step
+
+Then it will be required to modify functions that works with types. When
+typtype of a retrieved type is 'd' then it will perform lookups into
+pg_class, pg_attribute and pg_attrdef to find the real definition of the
+domain. These additional lookups will also create a performace penalty
+of using domains. But every feature has its costs. I know this paragraph
+about the real implementation is very short, but I think there are
+people that know the "type mechanism" better then I know. And can easier
+tell if it is possible to go this way.
+
+I hope you understand my explanation. It is also possible that I don't
+know some aspects of the backend code that makes my idea wrong.
+
+ Dan
+
+----------------------------------------------
+Ing. Daniel Horak
+network and system administrator
+e-mail: horak@sit.plzen-city.cz
+privat e-mail: dan.horak@email.cz ICQ:36448176
+----------------------------------------------
+
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+
+From pgsql-hackers-owner+M8925@postgresql.org Thu May 17 14:04:29 2001
+Return-path: <pgsql-hackers-owner+M8925@postgresql.org>
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+ Thu, 17 May 2001 13:58:45 -0400 (EDT)
+To: =?iso-8859-2?Q?Hor=E1k_Daniel?= <horak@sit.plzen-city.cz>
+cc: pgsql-hackers@postgresql.org
+Subject: Re: [HACKERS] possible DOMAIN implementation
+In-Reply-To: <9C8918CC8CDAAC4AA79D4594A480648901E68C2B@EXCHANGE.mmp.plzen-city.cz>
+References: <9C8918CC8CDAAC4AA79D4594A480648901E68C2B@EXCHANGE.mmp.plzen-city.cz>
+Comments: In-reply-to =?iso-8859-2?Q?Hor=E1k_Daniel?= <horak@sit.plzen-city.cz>
+ message dated "Thu, 17 May 2001 17:22:05 +0200"
+Date: Thu, 17 May 2001 13:58:45 -0400
+Message-ID: <12167.990122325@sss.pgh.pa.us>
+From: Tom Lane <tgl@sss.pgh.pa.us>
+Precedence: bulk
+Sender: pgsql-hackers-owner@postgresql.org
+Status: OR
+
+=?iso-8859-2?Q?Hor=E1k_Daniel?= <horak@sit.plzen-city.cz> writes:
+> When a new domain is created it will:
+> - put a record into pg_type with typnam = domain name, new code for
+> typtype = 'd' and typrelid = oid of a new record in pg_class (next line)
+> - put a record into pg_class to create a fictional table with a new
+> relkind ('d'?), relnatts = 1, relname can be system generated
+> (pg_d_<domainname>)
+
+Ugh. Don't overload pg_class with things that are not tables. I see no
+reason that either pg_class or pg_attribute should be involved in the
+definition of a domain. Make new system tables if you need to, but
+don't confuse the semantics of critical tables.
+
+> - put a records into pg_attribute and pg_attrdef with "column
+> (attribute) definition" - real type, size, default value etc., owner
+> will the fictional table from the previous step
+
+> Then it will be required to modify functions that works with types. When
+> typtype of a retrieved type is 'd' then it will perform lookups into
+> pg_class, pg_attribute and pg_attrdef to find the real definition of the
+> domain. These additional lookups will also create a performace penalty
+> of using domains.
+
+Why shouldn't this info be directly available from the pg_type row?
+
+ regards, tom lane
+
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