mirror of https://github.com/postgres/postgres
Bruce Momjian
25 years ago
parent
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@ -0,0 +1,722 @@ |
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From pgsql-hackers-owner+M3040@hub.org Thu Jun 8 00:31:01 2000 |
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From: "Hiroshi Inoue" <Inoue@tpf.co.jp> |
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To: "Bruce Momjian" <pgman@candle.pha.pa.us> |
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Cc: "PostgreSQL-development" <pgsql-hackers@postgreSQL.org> |
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Subject: RE: [HACKERS] DROP COLUMN status |
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Date: Thu, 8 Jun 2000 13:07:44 +0900 |
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|
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> -----Original Message----- |
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> From: pgsql-hackers-owner@hub.org [mailto:pgsql-hackers-owner@hub.org]On |
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> Behalf Of Bruce Momjian |
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> |
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> Can someone comment on where we are with DROP COLUMN? |
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> |
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|
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I've already committed my trial implementation 3 months ago. |
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They are $ifdef'd by _DROP_COLUMN_HACK__. |
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Please enable the feature and evaluate it. |
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You could enable the feature without initdb. |
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|
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Regards. |
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|
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Hiroshi Inoue |
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Inoue@tpf.co.jp |
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|
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|
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From Inoue@tpf.co.jp Thu Jun 8 02:03:27 2000 |
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id PAA01221; Thu, 08 Jun 2000 15:03:23 +0900 |
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From: "Hiroshi Inoue" <Inoue@tpf.co.jp> |
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To: "Bruce Momjian" <pgman@candle.pha.pa.us> |
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Cc: "PostgreSQL-development" <pgsql-hackers@postgreSQL.org> |
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Subject: RE: [HACKERS] DROP COLUMN status |
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Date: Thu, 8 Jun 2000 15:05:24 +0900 |
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|
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> -----Original Message----- |
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> From: Bruce Momjian [mailto:pgman@candle.pha.pa.us] |
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> Sent: Thursday, June 08, 2000 1:58 PM |
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> |
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> [ Charset ISO-8859-1 unsupported, converting... ] |
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> > > -----Original Message----- |
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> > > From: pgsql-hackers-owner@hub.org |
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> [mailto:pgsql-hackers-owner@hub.org]On |
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> > > Behalf Of Bruce Momjian |
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> > > |
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> > > Can someone comment on where we are with DROP COLUMN? |
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> > > |
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> > |
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> > I've already committed my trial implementation 3 months ago. |
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> > They are $ifdef'd by _DROP_COLUMN_HACK__. |
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> > Please enable the feature and evaluate it. |
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> > You could enable the feature without initdb. |
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> |
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> OK, can you explain how it works, and add any needed documentation so we |
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> can enable it. |
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> |
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|
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First it's only a trial so I don't implement it completely. |
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Especially I don't completely drop related objects |
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(FK_constraint,triggers,views etc). I don't know whether |
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we could drop them properly or not. |
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|
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The implementation makes the dropped column invisible by |
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changing its attnum to -attnum - offset(currently 20) and |
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attnam to ("*already Dropped%d",attnum). It doesn't touch |
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the table at all. After dropping a column insert/update |
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operation regards the column as NULL and other related |
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stuff simply ignores the column. |
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|
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Regards. |
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|
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Hiroshi Inoue |
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Inoue@tpf.co.jp |
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|
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From tgl@sss.pgh.pa.us Thu Jun 8 10:20:34 2000 |
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To: "Hiroshi Inoue" <Inoue@tpf.co.jp> |
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cc: "Bruce Momjian" <pgman@candle.pha.pa.us>, |
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"PostgreSQL-development" <pgsql-hackers@postgresql.org> |
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Subject: Re: [HACKERS] DROP COLUMN status |
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In-reply-to: <000f01bfd10f$893798a0$2801007e@tpf.co.jp> |
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References: <000f01bfd10f$893798a0$2801007e@tpf.co.jp> |
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Comments: In-reply-to "Hiroshi Inoue" <Inoue@tpf.co.jp> |
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message dated "Thu, 08 Jun 2000 15:05:24 +0900" |
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Date: Thu, 08 Jun 2000 10:20:11 -0400 |
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Message-ID: <15722.960474011@sss.pgh.pa.us> |
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From: Tom Lane <tgl@sss.pgh.pa.us> |
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Status: ORr |
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|
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"Hiroshi Inoue" <Inoue@tpf.co.jp> writes: |
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> The implementation makes the dropped column invisible by |
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> changing its attnum to -attnum - offset(currently 20) and |
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> attnam to ("*already Dropped%d",attnum). |
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|
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Ugh. No wonder you had to hack so many places in such an ugly fashion. |
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Why not leave the attnum as-is, and just add a bool saying "column is |
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dropped" to pg_attribute? As long as the parser ignores columns marked |
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that way for field lookup and expansion of *, it seems the rest of the |
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system wouldn't need to treat dropped columns specially in any way. |
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|
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regards, tom lane |
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|
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From pgsql-hackers-owner+M3094@hub.org Thu Jun 8 15:58:30 2000 |
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Thu, 8 Jun 2000 15:52:43 -0400 (EDT) |
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To: Bruce Momjian <pgman@candle.pha.pa.us> |
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cc: Hiroshi Inoue <Inoue@tpf.co.jp>, |
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PostgreSQL-development <pgsql-hackers@postgreSQL.org> |
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Subject: Re: [HACKERS] DROP COLUMN status |
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In-reply-to: <200006081541.LAA01566@candle.pha.pa.us> |
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References: <200006081541.LAA01566@candle.pha.pa.us> |
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Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us> |
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message dated "Thu, 08 Jun 2000 11:41:43 -0400" |
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Date: Thu, 08 Jun 2000 15:52:43 -0400 |
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Message-ID: <19687.960493963@sss.pgh.pa.us> |
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From: Tom Lane <tgl@sss.pgh.pa.us> |
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|
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>>>> The implementation makes the dropped column invisible by |
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>>>> changing its attnum to -attnum - offset(currently 20) and |
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>>>> attnam to ("*already Dropped%d",attnum). |
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>> |
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>> Ugh. No wonder you had to hack so many places in such an ugly fashion. |
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>> Why not leave the attnum as-is, and just add a bool saying "column is |
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>> dropped" to pg_attribute? As long as the parser ignores columns marked |
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>> that way for field lookup and expansion of *, it seems the rest of the |
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>> system wouldn't need to treat dropped columns specially in any way. |
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|
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> If we leave it as positive, don't we have to change user applications |
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> that query pg_attribute so they also know to skip it? |
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|
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Good point, but I think user applications that query pg_attribute |
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are likely to have trouble anyway: if they're expecting a consecutive |
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series of attnums then they're going to lose no matter what. |
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|
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regards, tom lane |
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|
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From hannu@tm.ee Sat Jun 10 01:02:57 2000 |
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To: Bruce Momjian <pgman@candle.pha.pa.us> |
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CC: Tom Lane <tgl@sss.pgh.pa.us>, Peter Eisentraut <peter_e@gmx.net>, |
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PostgreSQL Development <pgsql-hackers@postgresql.org> |
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Subject: Re: [HACKERS] ALTER TABLE DROP COLUMN |
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References: <200006091249.IAA18730@candle.pha.pa.us> |
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Content-Type: text/plain; charset=us-ascii |
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Status: OR |
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|
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Bruce Momjian wrote: |
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> |
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> Seems we have 4 DROP COLUMN ideas: |
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> |
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> Method Advantage |
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> ----------------------------------------------------------------- |
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> 1 invisible column marked by negative attnum fast |
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> 2 invisible column marked by is_dropped column fast |
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> 3 make copy of table without column col removed |
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> 4 make new tuples in existing table without column col removed |
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|
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IIRC there was a fifth idea, a variation of 2 that would work better |
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with |
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inheritance - |
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|
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5 all columns have is_real_column attribute that is true for all |
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coluns |
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present in that relation, so situations like |
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|
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create table tab_a(a_i int); |
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create table tab_b(b_i int) inherits(tab_a); |
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alter table tab_a add column c_i int; |
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|
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can be made to work. |
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|
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It would also require clients to ignore all missing columns that backend |
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can |
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pass to them as nulls (which is usually quite cheap in bandwith usage) |
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in |
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case of "SELECT **" queries. |
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|
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We could even rename attno to attid to make folks aware that it is not |
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be |
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assumed to be continuous. |
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|
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> Folks, we had better choose one and get started. |
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> |
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> Number 1 Hiroshi has ifdef'ed out in the code. Items 1 and 2 have |
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> problems with backend code and 3rd party code not seeing the dropped |
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> columns, or having gaps in the attno numbering. |
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|
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If we want to make ADD COLUMN to work with inheritance wihout having to |
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rewrite every single tuple in both parent and inherited tables, we will |
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have to accept the fact that there are caps in in attno numbering. |
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|
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> Number 3 has problems |
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> with making it an atomic operation, and number 4 is described below. |
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|
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Nr 4 has still problems with attno numbering _changing_ during drop |
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which |
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could either be better or worse for client software than having gaps - |
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in both cases client must be prepared to deal with runtime changes in |
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attribute definition. |
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|
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-------------- |
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Hannu |
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|
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From Inoue@tpf.co.jp Sat Jun 10 01:01:01 2000 |
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From: "Hiroshi Inoue" <Inoue@tpf.co.jp> |
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To: "Bruce Momjian" <pgman@candle.pha.pa.us>, "Tom Lane" <tgl@sss.pgh.pa.us> |
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Cc: "Peter Eisentraut" <peter_e@gmx.net>, |
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"PostgreSQL Development" <pgsql-hackers@postgreSQL.org> |
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Subject: RE: [HACKERS] ALTER TABLE DROP COLUMN |
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Date: Sat, 10 Jun 2000 13:43:26 +0900 |
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Message-ID: <EKEJJICOHDIEMGPNIFIJEELACBAA.Inoue@tpf.co.jp> |
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Status: ORr |
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|
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> -----Original Message----- |
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> From: pgsql-hackers-owner@hub.org |
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> [mailto:pgsql-hackers-owner@hub.org]On Behalf Of Bruce Momjian |
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> |
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> Seems we have 4 DROP COLUMN ideas: |
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> |
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> Method Advantage |
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> ----------------------------------------------------------------- |
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> 1 invisible column marked by negative attnum fast |
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> 2 invisible column marked by is_dropped column fast |
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> 3 make copy of table without column col removed |
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> 4 make new tuples in existing table without column col removed |
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> |
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> Folks, we had better choose one and get started. |
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> |
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> Number 1 Hiroshi has ifdef'ed out in the code. Items 1 and 2 have |
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> problems with backend code and 3rd party code not seeing the dropped |
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> columns, |
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|
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Hmm,doesn't *not seeing* mean the column is dropped ? |
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|
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> or having gaps in the attno numbering. Number 3 has problems |
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> with making it an atomic operation, and number 4 is described below. |
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> |
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|
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Don't forget another important point. |
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|
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Currently even DROP TABLE doesn't remove related objects completely. |
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And I don't think I could remove objects related to the dropping column |
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completely using 1)2) in ALTER TABLE DROP COLUMN implementation. |
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|
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Using 3)4) we should not only remove objects as 1)2) but also |
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change attnum-s in all objects related to the relation. Otherwise |
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PostgreSQL would do the wrong thing silently. |
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|
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Regards. |
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|
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Hiroshi Inoue |
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Inoue@tpf.co.jp |
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|
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From dhogaza@pacifier.com Sat Jun 10 01:01:06 2000 |
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Date: Fri, 09 Jun 2000 21:57:58 -0700 |
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To: "Hiroshi Inoue" <Inoue@tpf.co.jp>, |
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"Bruce Momjian" <pgman@candle.pha.pa.us>, |
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"Tom Lane" <tgl@sss.pgh.pa.us> |
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From: Don Baccus <dhogaza@pacifier.com> |
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Subject: RE: [HACKERS] ALTER TABLE DROP COLUMN |
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Cc: "Peter Eisentraut" <peter_e@gmx.net>, |
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"PostgreSQL Development" <pgsql-hackers@postgreSQL.org> |
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In-Reply-To: <EKEJJICOHDIEMGPNIFIJEELACBAA.Inoue@tpf.co.jp> |
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References: <200006091249.IAA18730@candle.pha.pa.us> |
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Status: OR |
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|
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At 01:43 PM 6/10/00 +0900, Hiroshi Inoue wrote: |
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>> -----Original Message----- |
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>> From: pgsql-hackers-owner@hub.org |
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>> [mailto:pgsql-hackers-owner@hub.org]On Behalf Of Bruce Momjian |
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>> |
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>> Seems we have 4 DROP COLUMN ideas: |
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>> |
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>> Method Advantage |
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>> ----------------------------------------------------------------- |
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>> 1 invisible column marked by negative attnum fast |
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>> 2 invisible column marked by is_dropped column fast |
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>> 3 make copy of table without column col removed |
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>> 4 make new tuples in existing table without column col removed |
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>> |
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>> Folks, we had better choose one and get started. |
||||
|
||||
Oracle gives you the choice between the "cheating" fast method(s) and |
||||
the "real" slow (really slow?) real method. |
||||
|
||||
So there's at least real world experience by virtue of example by |
||||
the world's most successful database supplier that user control |
||||
over "hide the column" and "really delete the column" is valuable. |
||||
|
||||
It really makes a lot of sense to give such a choice. If one |
||||
does so by "hiding", at a later date one would think the choice |
||||
of "really deleting" would be a possibility. I don't know if |
||||
Oracle does this... |
||||
|
||||
If not, they might not care. In today's world, there are bazillions |
||||
of dollars for Oracle to scoop up from users who could just as easily |
||||
be PG users - all those "we'll fail if don't IPO 'cause we'll never |
||||
have any customers" database-backed websites :) |
||||
|
||||
|
||||
|
||||
- Don Baccus, Portland OR <dhogaza@pacifier.com> |
||||
Nature photos, on-line guides, Pacific Northwest |
||||
Rare Bird Alert Service and other goodies at |
||||
http://donb.photo.net. |
||||
|
||||
From tgl@sss.pgh.pa.us Sat Jun 10 01:31:04 2000 |
||||
Received: from renoir.op.net (root@renoir.op.net [207.29.195.4]) |
||||
by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id BAA10922 |
||||
for <pgman@candle.pha.pa.us>; Sat, 10 Jun 2000 01:31:03 -0400 (EDT) |
||||
Received: from sss2.sss.pgh.pa.us (sss.pgh.pa.us [209.114.166.2]) by renoir.op.net (o1/$Revision: 1.1 $) with ESMTP id BAA27265 for <pgman@candle.pha.pa.us>; Sat, 10 Jun 2000 01:16:07 -0400 (EDT) |
||||
Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1]) |
||||
by sss2.sss.pgh.pa.us (8.9.3/8.9.3) with ESMTP id BAA06206; |
||||
Sat, 10 Jun 2000 01:14:37 -0400 (EDT) |
||||
To: Don Baccus <dhogaza@pacifier.com> |
||||
cc: "Hiroshi Inoue" <Inoue@tpf.co.jp>, |
||||
"Bruce Momjian" <pgman@candle.pha.pa.us>, |
||||
"Peter Eisentraut" <peter_e@gmx.net>, |
||||
"PostgreSQL Development" <pgsql-hackers@postgreSQL.org> |
||||
Subject: Re: [HACKERS] ALTER TABLE DROP COLUMN |
||||
In-reply-to: <3.0.1.32.20000609215758.0116d850@mail.pacifier.com> |
||||
References: <200006091249.IAA18730@candle.pha.pa.us> <3.0.1.32.20000609215758.0116d850@mail.pacifier.com> |
||||
Comments: In-reply-to Don Baccus <dhogaza@pacifier.com> |
||||
message dated "Fri, 09 Jun 2000 21:57:58 -0700" |
||||
Date: Sat, 10 Jun 2000 01:14:37 -0400 |
||||
Message-ID: <6203.960614077@sss.pgh.pa.us> |
||||
From: Tom Lane <tgl@sss.pgh.pa.us> |
||||
Status: OR |
||||
|
||||
Don Baccus <dhogaza@pacifier.com> writes: |
||||
> Oracle gives you the choice between the "cheating" fast method(s) and |
||||
> the "real" slow (really slow?) real method. |
||||
|
||||
> So there's at least real world experience by virtue of example by |
||||
> the world's most successful database supplier that user control |
||||
> over "hide the column" and "really delete the column" is valuable. |
||||
|
||||
Sure, but you don't need any help from the database to do "really delete |
||||
the column". SELECT INTO... is enough, and it's not even any slower |
||||
than the implementations under discussion. |
||||
|
||||
So I'm satisfied if we offer the "hide the column" approach. |
||||
|
||||
Has anyone thought about what happens to table constraints that use the |
||||
doomed column? Triggers, RI rules, yadda yadda? |
||||
|
||||
Has anyone thought about undoing a DELETE COLUMN? The data is still |
||||
there, at least in tuples that have not been updated, so it's not |
||||
totally unreasonable. |
||||
|
||||
regards, tom lane |
||||
|
||||
From dhogaza@pacifier.com Sat Jun 10 09:30:59 2000 |
||||
Received: from renoir.op.net (root@renoir.op.net [207.29.195.4]) |
||||
by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id JAA25987 |
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for <pgman@candle.pha.pa.us>; Sat, 10 Jun 2000 09:30:58 -0400 (EDT) |
||||
Received: from smtp.pacifier.com (comet.pacifier.com [199.2.117.155]) by renoir.op.net (o1/$Revision: 1.1 $) with ESMTP id JAA18716 for <pgman@candle.pha.pa.us>; Sat, 10 Jun 2000 09:15:08 -0400 (EDT) |
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Received: from desktop (dsl-dhogaza.pacifier.net [207.202.226.68]) |
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by smtp.pacifier.com (8.9.3/8.9.3pop) with SMTP id GAA15799; |
||||
Sat, 10 Jun 2000 06:14:28 -0700 (PDT) |
||||
Message-Id: <3.0.1.32.20000610054306.0115f020@mail.pacifier.com> |
||||
X-Sender: dhogaza@mail.pacifier.com |
||||
X-Mailer: Windows Eudora Pro Version 3.0.1 (32) |
||||
Date: Sat, 10 Jun 2000 05:43:06 -0700 |
||||
To: Tom Lane <tgl@sss.pgh.pa.us> |
||||
From: Don Baccus <dhogaza@pacifier.com> |
||||
Subject: Re: [HACKERS] ALTER TABLE DROP COLUMN |
||||
Cc: "Hiroshi Inoue" <Inoue@tpf.co.jp>, |
||||
"Bruce Momjian" <pgman@candle.pha.pa.us>, |
||||
"Peter Eisentraut" <peter_e@gmx.net>, |
||||
"PostgreSQL Development" <pgsql-hackers@postgreSQL.org> |
||||
In-Reply-To: <6203.960614077@sss.pgh.pa.us> |
||||
References: <3.0.1.32.20000609215758.0116d850@mail.pacifier.com> |
||||
<200006091249.IAA18730@candle.pha.pa.us> |
||||
<3.0.1.32.20000609215758.0116d850@mail.pacifier.com> |
||||
Mime-Version: 1.0 |
||||
Content-Type: text/plain; charset="us-ascii" |
||||
Status: OR |
||||
|
||||
At 01:14 AM 6/10/00 -0400, Tom Lane wrote: |
||||
>Don Baccus <dhogaza@pacifier.com> writes: |
||||
>> Oracle gives you the choice between the "cheating" fast method(s) and |
||||
>> the "real" slow (really slow?) real method. |
||||
> |
||||
>> So there's at least real world experience by virtue of example by |
||||
>> the world's most successful database supplier that user control |
||||
>> over "hide the column" and "really delete the column" is valuable. |
||||
> |
||||
>Sure, but you don't need any help from the database to do "really delete |
||||
>the column". SELECT INTO... is enough, and it's not even any slower |
||||
>than the implementations under discussion. |
||||
> |
||||
>So I'm satisfied if we offer the "hide the column" approach. |
||||
|
||||
<shrug> I wouldn't put a "real" drop column at the top of my list |
||||
of priorities, but there is something to be said for user convenience. |
||||
|
||||
|
||||
|
||||
- Don Baccus, Portland OR <dhogaza@pacifier.com> |
||||
Nature photos, on-line guides, Pacific Northwest |
||||
Rare Bird Alert Service and other goodies at |
||||
http://donb.photo.net. |
||||
|
||||
From tgl@sss.pgh.pa.us Sun Jun 11 12:31:03 2000 |
||||
Received: from renoir.op.net (root@renoir.op.net [207.29.195.4]) |
||||
by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id MAA05771 |
||||
for <pgman@candle.pha.pa.us>; Sun, 11 Jun 2000 12:31:01 -0400 (EDT) |
||||
Received: from sss2.sss.pgh.pa.us (sss.pgh.pa.us [209.114.166.2]) by renoir.op.net (o1/$Revision: 1.1 $) with ESMTP id MAA19315 for <pgman@candle.pha.pa.us>; Sun, 11 Jun 2000 12:24:06 -0400 (EDT) |
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Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1]) |
||||
by sss2.sss.pgh.pa.us (8.9.3/8.9.3) with ESMTP id MAA09503; |
||||
Sun, 11 Jun 2000 12:22:42 -0400 (EDT) |
||||
To: "Hiroshi Inoue" <Inoue@tpf.co.jp> |
||||
cc: "Bruce Momjian" <pgman@candle.pha.pa.us>, |
||||
"Peter Eisentraut" <peter_e@gmx.net>, |
||||
"PostgreSQL Development" <pgsql-hackers@postgreSQL.org> |
||||
Subject: Re: [HACKERS] ALTER TABLE DROP COLUMN |
||||
In-reply-to: <EKEJJICOHDIEMGPNIFIJEELACBAA.Inoue@tpf.co.jp> |
||||
References: <EKEJJICOHDIEMGPNIFIJEELACBAA.Inoue@tpf.co.jp> |
||||
Comments: In-reply-to "Hiroshi Inoue" <Inoue@tpf.co.jp> |
||||
message dated "Sat, 10 Jun 2000 13:43:26 +0900" |
||||
Date: Sun, 11 Jun 2000 12:22:42 -0400 |
||||
Message-ID: <9500.960740562@sss.pgh.pa.us> |
||||
From: Tom Lane <tgl@sss.pgh.pa.us> |
||||
Status: ORr |
||||
|
||||
>> Seems we have 4 DROP COLUMN ideas: |
||||
>> Method Advantage |
||||
>> ----------------------------------------------------------------- |
||||
>> 1 invisible column marked by negative attnum fast |
||||
>> 2 invisible column marked by is_dropped column fast |
||||
>> 3 make copy of table without column col removed |
||||
>> 4 make new tuples in existing table without column col removed |
||||
|
||||
Bruce and I talked about this by phone yesterday, and we realized that |
||||
none of these are very satisfactory. #1 and #2 both have the flaw that |
||||
applications that examine pg_attribute will probably break: they will |
||||
see a sequence of attnum values with gaps in it. And what should the |
||||
rel's relnatts field be set to? #3 and #4 are better on that point, |
||||
but they leave us with the problem of renumbering references to columns |
||||
after the dropped one in constraints, rules, PL functions, etc. |
||||
|
||||
Furthermore, there is a closely related problem that none of these |
||||
approaches give us much help on: recursive ALTER TABLE ADD COLUMN. |
||||
Right now, ADD puts the new column at the end of each table it's added |
||||
to, which often means that it gets a different column number in child |
||||
tables than in parent tables. That leads to havoc for pg_dump. |
||||
|
||||
I think the only clean solution is to create a clear distinction between |
||||
physical and logical column numbers. Each pg_attribute tuple would need |
||||
two attnum fields, and pg_class would need two relnatts fields as well. |
||||
A column once created would never change its physical column number, but |
||||
its logical column number might change as a consequence of adding or |
||||
dropping columns before it. ADD COLUMN would ensure that a column added |
||||
to child tables receives the same logical column number as it has in the |
||||
parent table, thus solving the dump/reload problem. DROP COLUMN would |
||||
assign an invalid logical column number to dropped columns. They could |
||||
be numbered zero except that we'd probably still want a unique index on |
||||
attrelid+attnum, and the index would complain. I'd suggest using |
||||
Hiroshi's idea: give a dropped column a logical attnum equal to |
||||
-(physical_attnum + offset). |
||||
|
||||
With this approach, internal operations on tuples would all use |
||||
physical column numbers, but operations that interface to the outside |
||||
world would present a view of only the valid logical columns. For |
||||
example, the parser would only allow logical columns to be referenced |
||||
by name; "SELECT *" would expand to valid logical columns in logical- |
||||
column-number order; COPY would send or receive valid logical columns |
||||
in logical-column-number order; etc. |
||||
|
||||
Stored rules and so forth probably should store physical column numbers |
||||
so that they need not be modified during column add/drop. |
||||
|
||||
This would require looking at all the places in the backend to determine |
||||
whether they should be working with logical or physical column numbers, |
||||
but the design is such that most all places would want to be using |
||||
physical numbers, so I don't think it'd be too painful. |
||||
|
||||
Although I'd prefer to give the replacement columns two new names |
||||
(eg, "attlnum" and "attpnum") to ensure we find all uses, this would |
||||
surely break applications that examine pg_attribute. For compatibility |
||||
we'd have to recycle "attnum" and "relnatts" to indicate logical column |
||||
number and logical column count, while adding new fields (say "attpnum" |
||||
and "relnpatts") for the physical number and count. |
||||
|
||||
Comments? |
||||
|
||||
regards, tom lane |
||||
|
||||
From pgsql-hackers-owner+M3184@hub.org Mon Jun 12 09:29:17 2000 |
||||
Received: from hub.org (root@hub.org [216.126.84.1]) |
||||
by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id JAA16538 |
||||
for <pgman@candle.pha.pa.us>; Mon, 12 Jun 2000 09:29:15 -0400 (EDT) |
||||
Received: from hub.org (majordom@localhost [127.0.0.1]) |
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by hub.org (8.10.1/8.10.1) with SMTP id e5C9RxT92685; |
||||
Mon, 12 Jun 2000 05:27:59 -0400 (EDT) |
||||
Received: from clio.trends.ca (root@clio.trends.ca [209.47.148.2]) |
||||
by hub.org (8.10.1/8.10.1) with ESMTP id e5C8YWT89945 |
||||
for <pgsql-hackers@postgreSQL.org>; Mon, 12 Jun 2000 04:34:32 -0400 (EDT) |
||||
Received: from sd.tpf.co.jp (sd.tpf.co.jp [210.161.239.34]) |
||||
by clio.trends.ca (8.9.3+Sun/8.9.3) with ESMTP id VAA17711 |
||||
for <pgsql-hackers@postgreSQL.org>; Sun, 11 Jun 2000 21:40:28 -0400 (EDT) |
||||
Received: from cadzone ([126.0.1.40] (may be forged)) |
||||
by sd.tpf.co.jp (2.5 Build 2640 (Berkeley 8.8.6)/8.8.4) with SMTP |
||||
id KAA03734; Mon, 12 Jun 2000 10:38:42 +0900 |
||||
From: "Hiroshi Inoue" <Inoue@tpf.co.jp> |
||||
To: "Tom Lane" <tgl@sss.pgh.pa.us> |
||||
Cc: "Bruce Momjian" <pgman@candle.pha.pa.us>, |
||||
"Peter Eisentraut" <peter_e@gmx.net>, |
||||
"PostgreSQL Development" <pgsql-hackers@postgresql.org> |
||||
Subject: RE: [HACKERS] ALTER TABLE DROP COLUMN |
||||
Date: Mon, 12 Jun 2000 10:40:47 +0900 |
||||
Message-ID: <000b01bfd40f$3b3091e0$2801007e@tpf.co.jp> |
||||
MIME-Version: 1.0 |
||||
Content-Type: text/plain; |
||||
charset="iso-2022-jp" |
||||
Content-Transfer-Encoding: 7bit |
||||
X-Priority: 3 (Normal) |
||||
X-MSMail-Priority: Normal |
||||
X-Mailer: Microsoft Outlook 8.5, Build 4.71.2173.0 |
||||
In-Reply-To: <9500.960740562@sss.pgh.pa.us> |
||||
X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2314.1300 |
||||
Importance: Normal |
||||
X-Mailing-List: pgsql-hackers@postgresql.org |
||||
Precedence: bulk |
||||
Sender: pgsql-hackers-owner@hub.org |
||||
Status: OR |
||||
|
||||
> -----Original Message----- |
||||
> From: Tom Lane [mailto:tgl@sss.pgh.pa.us] |
||||
> |
||||
> >> Seems we have 4 DROP COLUMN ideas: |
||||
> >> Method Advantage |
||||
> >> ----------------------------------------------------------------- |
||||
> >> 1 invisible column marked by negative attnum fast |
||||
> >> 2 invisible column marked by is_dropped column fast |
||||
> >> 3 make copy of table without column col removed |
||||
> >> 4 make new tuples in existing table without column col removed |
||||
> |
||||
|
||||
Hmm,I've received no pg-ML mails for more than 1 day. |
||||
What's happened with pgsql ML ? |
||||
|
||||
> Bruce and I talked about this by phone yesterday, and we realized that |
||||
> none of these are very satisfactory. #1 and #2 both have the flaw that |
||||
> applications that examine pg_attribute will probably break: they will |
||||
> see a sequence of attnum values with gaps in it. And what should the |
||||
> rel's relnatts field be set to? #3 and #4 are better on that point, |
||||
> but they leave us with the problem of renumbering references to columns |
||||
> after the dropped one in constraints, rules, PL functions, etc. |
||||
> |
||||
> Furthermore, there is a closely related problem that none of these |
||||
> approaches give us much help on: recursive ALTER TABLE ADD COLUMN. |
||||
> Right now, ADD puts the new column at the end of each table it's added |
||||
> to, which often means that it gets a different column number in child |
||||
> tables than in parent tables. That leads to havoc for pg_dump. |
||||
> |
||||
|
||||
Inheritance is one of the reason why I didn't take #2. I don't understand |
||||
marking is_dropped is needed or not when pg_attribute is overhauled |
||||
for inheritance. |
||||
I myself have never wanted to use current inheritance functionality |
||||
mainly because of this big flaw. Judging from the recent discussion |
||||
about oo(though I don't understand details),the change seems to be |
||||
needed in order to make inheritance functionality really useful. |
||||
|
||||
> I think the only clean solution is to create a clear distinction between |
||||
> physical and logical column numbers. Each pg_attribute tuple would need |
||||
> two attnum fields, and pg_class would need two relnatts fields as well. |
||||
> A column once created would never change its physical column number, but |
||||
|
||||
I don't understand inheritance well. In the near future wouldn't the |
||||
implementation require e.g. attid which is common to all children |
||||
of a parent and is never changed ? If so,we would need the third |
||||
attid field which is irrevalent to physical/logical position. If not, |
||||
physical column number would be sufficient . |
||||
|
||||
> its logical column number might change as a consequence of adding or |
||||
> dropping columns before it. ADD COLUMN would ensure that a column added |
||||
> to child tables receives the same logical column number as it has in the |
||||
> parent table, thus solving the dump/reload problem. DROP COLUMN would |
||||
> assign an invalid logical column number to dropped columns. They could |
||||
> be numbered zero except that we'd probably still want a unique index on |
||||
> attrelid+attnum, and the index would complain. I'd suggest using |
||||
> Hiroshi's idea: give a dropped column a logical attnum equal to |
||||
> -(physical_attnum + offset). |
||||
> |
||||
> With this approach, internal operations on tuples would all use |
||||
> physical column numbers, but operations that interface to the outside |
||||
> world would present a view of only the valid logical columns. For |
||||
> example, the parser would only allow logical columns to be referenced |
||||
> by name; "SELECT *" would expand to valid logical columns in logical- |
||||
> column-number order; COPY would send or receive valid logical columns |
||||
> in logical-column-number order; etc. |
||||
> |
||||
> Stored rules and so forth probably should store physical column numbers |
||||
> so that they need not be modified during column add/drop. |
||||
> |
||||
> This would require looking at all the places in the backend to determine |
||||
> whether they should be working with logical or physical column numbers, |
||||
> but the design is such that most all places would want to be using |
||||
> physical numbers, so I don't think it'd be too painful. |
||||
> |
||||
> Although I'd prefer to give the replacement columns two new names |
||||
> (eg, "attlnum" and "attpnum") to ensure we find all uses, this would |
||||
> surely break applications that examine pg_attribute. For compatibility |
||||
> we'd have to recycle "attnum" and "relnatts" to indicate logical column |
||||
> number and logical column count, while adding new fields (say "attpnum" |
||||
> and "relnpatts") for the physical number and count. |
||||
> |
||||
|
||||
I agree with you that we would add attpnum and change the meaing of |
||||
attnum as logical column number for backward compatibility. |
||||
|
||||
Regards. |
||||
|
||||
Hiroshi Inoue |
||||
Inoue@tpf.co.jp |
||||
|
||||
Loading…
Reference in new issue