From: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
To: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: ALTER TABLE on system catalogs
Date: Wed, 27 Jun 2018 22:31:30 +0200
Message-ID: <e49f825b-fb25-0bc8-8afc-d5ad895c7975@2ndquadrant.com> (raw)
ALTER TABLE on system catalogs is occasionally useful, for example
ALTER TABLE pg_attribute SET (autovacuum_vacuum_scale_factor=0);
However, this doesn't actually work. The above command produces
ERROR: AccessExclusiveLock required to add toast table.
If it's a shared catalog, it will produce
ERROR: shared tables cannot be toasted after initdb
In other cases it will work but then silently add a TOAST table to a
catalog, which I think we don't want.
The problem is that for (almost) any ALTER TABLE command, it afterwards
checks if the just-altered table now needs a TOAST table and tries to
add it if so, which will either fail or add a TOAST table that we don't
want.
I propose that we instead silently ignore attempts to add TOAST tables
to shared and system catalogs after bootstrapping. This fixes the above
issues. I have attached a patch for this, and also a test that
enshrines which system catalogs are supposed to have TOAST tables.
As an alternative, I tried to modify the ALTER TABLE code to avoid the
try-to-add-TOAST-table path depending on what ALTER TABLE actions were
done, but that seemed incredibly more complicated and harder to maintain
in the long run.
(You still need allow_system_table_mods=on for all of this. Perhaps
that's also worth revisiting, but it's a separate issue.)
--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
Attachments:download all (2 files, .tar.gz)0001-Add-test-for-system-catalog-TOAST-tables.patch (text/plain, 1.9K) download
From fa75f6573a37f4065f0ba16b1ef8a55e8b933af6 Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter_e@gmx.net>
Date: Fri, 18 May 2018 19:07:53 -0400
Subject: [PATCH 1/5] Add test for system catalog TOAST tables
This records which system catalogs are supposed to have a TOAST table.
---
src/test/regress/expected/misc_sanity.out | 22 ++++++++++++++++++++++
src/test/regress/sql/misc_sanity.sql | 10 ++++++++++
2 files changed, 32 insertions(+)
diff --git a/src/test/regress/expected/misc_sanity.out b/src/test/regress/expected/misc_sanity.outindex 5aaae6c39f..a1ea485a15 100644--- a/src/test/regress/expected/misc_sanity.out+++ b/src/test/regress/expected/misc_sanity.out@@ -76,3 +76,25 @@ NOTICE: pg_database contains unpinned initdb-created object(s)
NOTICE: pg_extension contains unpinned initdb-created object(s)
NOTICE: pg_rewrite contains unpinned initdb-created object(s)
NOTICE: pg_tablespace contains unpinned initdb-created object(s)
+-- **************** general ****************+-- check which system catalogs have toast tables+SELECT relname+FROM pg_class+WHERE relnamespace = 'pg_catalog'::regnamespace AND reltoastrelid <> 0+ORDER BY 1;+ relname +--------------------+ pg_attrdef+ pg_constraint+ pg_db_role_setting+ pg_description+ pg_proc+ pg_rewrite+ pg_seclabel+ pg_shdescription+ pg_shseclabel+ pg_statistic+ pg_statistic_ext+ pg_trigger+(12 rows)+diff --git a/src/test/regress/sql/misc_sanity.sql b/src/test/regress/sql/misc_sanity.sqlindex b921117fa5..b9adcbe4e0 100644--- a/src/test/regress/sql/misc_sanity.sql+++ b/src/test/regress/sql/misc_sanity.sql@@ -72,3 +72,13 @@
end if;
end loop;
end$$;
+++-- **************** general ****************++-- check which system catalogs have toast tables++SELECT relname+FROM pg_class+WHERE relnamespace = 'pg_catalog'::regnamespace AND reltoastrelid <> 0+ORDER BY 1;
base-commit: 2e61c50785a0dca6ed30a1a5522457b18bbb5498
--
2.18.0
0002-Ignore-attempts-to-add-TOAST-table-to-shared-or-cata.patch (text/plain, 2.0K) download
From 029fa7e97c8219f13b12ebd5026beb5ffd5ba684 Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter_e@gmx.net>
Date: Fri, 18 May 2018 17:05:27 -0400
Subject: [PATCH 2/5] Ignore attempts to add TOAST table to shared or catalog
tables
Running ALTER TABLE on any table will check if a TOAST table needs to be
added. On shared tables, this would previously fail, thus effectively
disabling ALTER TABLE for those tables. On (non-shared) system
catalogs, on the other hand, it would add a TOAST table, even though we
don't really want TOAST tables on some system catalogs. In some cases,
it would also fail with an error "AccessExclusiveLock required to add
toast table.", depending on what locks the ALTER TABLE actions had
already taken.
So instead, just ignore attempts to add TOAST tables to such tables,
outside of bootstrap mode, pretending they don't need one.
This allows running ALTER TABLE on such tables without messing up the
TOAST situation. (All this still requires allow_system_table_mods,
which is independent of this.)
---
src/backend/catalog/toasting.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/src/backend/catalog/toasting.c b/src/backend/catalog/toasting.cindex 3baaa08238..97a215d2c1 100644--- a/src/backend/catalog/toasting.c+++ b/src/backend/catalog/toasting.c@@ -17,6 +17,7 @@
#include "access/tuptoaster.h"
#include "access/xact.h"
#include "catalog/binary_upgrade.h"
+#include "catalog/catalog.h"
#include "catalog/dependency.h"
#include "catalog/heap.h"
#include "catalog/index.h"
@@ -153,9 +154,10 @@ create_toast_table(Relation rel, Oid toastOid, Oid toastIndexOid,
*/
shared_relation = rel->rd_rel->relisshared;
if (shared_relation && !IsBootstrapProcessingMode())
- ereport(ERROR,- (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),- errmsg("shared tables cannot be toasted after initdb")));+ return false;++ if (IsCatalogRelation(rel) && !IsBootstrapProcessingMode())+ return false;
/* It's mapped if and only if its parent is, too */
mapped_relation = RelationIsMapped(rel);
--
2.18.0
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To: pgsql-hackers@postgresql.org
Cc: peter.eisentraut@2ndquadrant.com
Subject: Re: ALTER TABLE on system catalogs
In-Reply-To: <e49f825b-fb25-0bc8-8afc-d5ad895c7975@2ndquadrant.com>
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