I am writing an idempotent schema change script for a Postgres 12 database. However I noticed that if I include the IF NOT EXISTS in an ADD COLUMN statement then even if the column already exists it is adding duplicate Indexes for the uniqueness constraint which already exists. Simple example:
-- set up base table
CREATE TABLE IF NOT EXISTS test_table
(id SERIAL PRIMARY KEY
);
-- statement intended to be idempotent
ALTER TABLE test_table
ADD COLUMN IF NOT EXISTS name varchar(50) UNIQUE;
Running this script creates a new index test_table_name_key[n] each time it is run. I can't find anything in the Postgres documentation and don't understand why this is allowed to happen? If I break it into two parts eg:
ALTER TABLE test_table
ADD COLUMN IF NOT EXISTS name varchar(50);
ALTER TABLE
ADD CONSTRAINT test_table_name_key UNIQUE (name);
Then the transaction fails because Postgres rejects the creation of a constraint which already exists (which I can then catch in a DO EXCEPTION block). As far as I can tell this is because doing it by this approach I am forced to give the constraint a name. This constrasts with the ALTER COLUMN SET NOT NULL which can be run multiple times without error or side effects as far as I can tell.
Question: why does it add a duplicate unique constraint and are there any problems with having multiple identical indexes on a table column? (I think this is a subtle 'error' and only spotted it by chance so am concerned it may arise in a production situation)
You can create multiple unique constraints on the same column as long as they have different names, simply because there is nothing in the PostgreSQL code that forbids that. Each unique constraint will create a unique index with the same name, because that is how unique constraints are implemented.
This can be a valid use case: for example, if the index is bloated, you could create a new constraint and then drop the old one.
But normally, it is useless and does harm, because each index will make data modifications on the table slower.
Related
I am recreating an existing table as a partitioned table in PostgreSQL 11.
After some research, I am approaching it using the following procedure so this can be done online while writes are still happening on the table:
add a check constraint on the existing table, first as not valid and then validating
drop the existing primary key
rename the existing table
create the partitioned table under the prior table name
attach the existing table as a partition to the new partitioned table
My expectation was that the last step would be relatively fast, but I don't really have a number for this. In my testing, it's taking about 30s. I wonder if my expectations are incorrect or if I'm doing something wrong with the constraint or anything else.
Here's a simplified version of the DDL.
First, the inserted_at column is declared like this:
inserted_at timestamp without time zone not null
I want to have an index on the ID even after I drop the PK for existing queries and writes, so I create an index:
create unique index concurrently my_events_temp_id_index on my_events (id);
The check constraint is created in one transaction:
alter table my_events add constraint my_events_2022_07_events_check
check (inserted_at >= '2018-01-01' and inserted_at < '2022-08-01')
not valid;
In the next transaction, it's validated (and the validation is successful):
alter table my_events validate constraint my_events_2022_07_events_check;
Then before creating the partitioned table, I drop the primary key of the existing table:
alter table my_events drop constraint my_events_pkey cascade;
Finally, in its own transaction, the partitioned table is created:
alter table my_events rename to my_events_2022_07;
create table my_events (
id uuid not null,
... other columns,
inserted_at timestamp without time zone not null,
primary key (id, inserted_at)
) partition by range (inserted_at);
alter table my_events attach partition my_events_2022_07
for values from ('2018-01-01') to ('2022-08-01');
That last transaction blocks inserts and takes about 30s for the 12M rows in my test database.
Edit
I wanted to add that in response to the attach I see this:
INFO: partition constraint for table "my_events_2022_07" is implied by existing constraints
That makes me think I'm doing this right.
The problem is not the check constraint, it is the primary key.
If you make the original unique index include both columns:
create unique index concurrently my_events_temp_id_index on my_events (id,inserted_at);
And if you make the new table have a unique index rather than a primary key on those two columns, then the attach is nearly instantaneous.
These seem to me like unneeded restrictions in PostgreSQL, both that the unique index on one column can't be used to imply uniqueness on the both columns, and that the unique index on both columns cannot be used to imply the primary key (nor even a unique constraint--but only a unique index).
I have some tables in PostgreSQL 12.9 that were declared as something like
-- This table is written in old style
create table old_style_table_1 (
id bigserial not null primary key,
...
);
-- This table uses new feature
create table new_style_table_2 (
id bigint generated by default as identity,
...
);
Second table seems to be declared using the identity flag introduced in 10th version.
Time went by, and we have partitioned the old tables, while keeping the original sequences:
CREATE TABLE partitioned_old_style_table_1 (LIKE old_style_table_1 INCLUDING DEFAULTS) PARTITION BY HASH (user_id);
CREATE TABLE partitioned_new_style_table_2 (LIKE new_style_table_2 INCLUDING DEFAULTS) PARTITION BY HASH (user_id);
DDL for their id columns seems to be id bigint default nextval('old_style_table_1_id_seq') not null and id bigint default nextval('new_style_table_2_id_seq') not null.
Everything has worked fine so far. Partitioned tables proved to be a great boon and we decided to retire the old tables by dropping them.
DROP TABLE old_style_table_1, new_style_table_2;
-- [2BP01] ERROR: cannot drop desired object(s) because other objects depend on them
-- Detail: default value for column id of table old_style_table_1 depends on sequence old_style_table_1_id_seq
-- default value for column id of table new_style_table_2 depends on sequence new_style_table_2_id_seq
After some pondering I've found out that sequences may have owners in postgres, so I opted to change them:
ALTER SEQUENCE old_style_table_1_id_seq OWNED BY partitioned_old_style_table_1.id;
DROP TABLE old_style_table_1;
-- Worked out flawlessly
ALTER SEQUENCE new_style_table_2_id_seq OWNED BY partitioned_new_style_table_2.id;
ALTER SEQUENCE new_style_table_2_id_seq OWNED BY NONE;
-- Here's the culprit of the question:
-- [0A000] ERROR: cannot change ownership of identity sequence
So, apparently the fact that this column has pg_attribute.attidentity set to 'd' forbids me from:
• changing the default value of the column:
ALTER TABLE new_style_table_2 ALTER COLUMN id SET DEFAULT 0;
-- [42601] ERROR: column "id" of relation "new_style_table_2" is an identity column
• dropping the default value:
ALTER TABLE new_style_table_2 ALTER COLUMN id DROP DEFAULT;
-- [42601] ERROR: column "id" of relation "new_style_table_2" is an identity column
-- Hint: Use ALTER TABLE ... ALTER COLUMN ... DROP IDENTITY instead.
• dropping the identity, column or the table altogether (new tables already depend on the sequence):
ALTER TABLE new_style_table_2 ALTER COLUMN id DROP IDENTITY IF EXISTS;
-- or
ALTER TABLE new_style_table_2 DROP COLUMN id;
-- or
DROP TABLE new_style_table_2;
-- result in
-- [2BP01] ERROR: cannot drop desired object(s) because other objects depend on them
-- default value for column id of table partitioned_new_style_table_2 depends on sequence new_style_table_2_id_seq
I've looked up the documentation, it provides the way to SET IDENTITY or ADD IDENTITY, but no way to remove it or to change to a throwaway sequence without attempting to drop the existing one.
➥ So, how am I able to remove an identity flag from the column-sequence pair so it won't affect other tables that use this sequence?
UPD: Tried running UPDATE pg_attribute SET attidentity='' WHERE attrelid=16816; on localhost, still receive [2BP01] and [0A000]. :/
Though I managed to execute the DROP DEFAULT value bit, but it seems like a dead end.
I don't think there is a safe and supported way to do that (without catalog modifications). Fortunately, there is nothing special about sequences that would make dropping them a problem. So take a short down time and:
remove the default value that uses the identity sequence
record the current value of the sequence
drop the table
create a new sequence with an appropriate START value
use the new sequence to set new default values
If you want an identity column, you should define it on the partitioned table, not on one of the partitions.
Let me start by saying that I’m a Linux/Unix admin. That being said my manager has tasked me with moving older PostgreSQL databases to a RedHat server running 8.4.20. I was successful moving a 7.2.1 db but I’m running into issues moving a 7.4.20 db.
I use pg_dump –c filename and psql < filename. For the problematic db everything runs until I get to a CREATE CONSTRAINT TRIGGER statement. If I run it as it is in the file I get :
NOTICE: ignoring incomplete trigger group for constraint "" FOREIGN KEY data(ups) REFERENCES upsinfo(ups)
DETAIL: Found referenced table's DELETE trigger.
CREATE TRIGGER
If I run set schema 'pg_catalog'; I get:
ERROR: relation "upsinfo" does not exist
The tables (I think) involved are:
CREATE TABLE upsinfo (
ups text NOT NULL,
ipaddr inet,
rcomm text,
wcomm text,
reachable boolean,
managed boolean,
comments text,
region text
);
CREATE TABLE data (
date timestamp with time zone,
ups text,
mib text,
value text
);
The trigger problem trigger statement:
CREATE CONSTRAINT TRIGGER "<unnamed>"
AFTER DELETE ON upsinfo
FROM data
NOT DEFERRABLE INITIALLY IMMEDIATE
FOR EACH ROW
EXECUTE PROCEDURE "RI_FKey_cascade_del"('<unnamed>', 'data', 'upsinfo', 'UNSPECIFIED', 'ups', 'ups');
I know that the RI_FKey_cascade_del function is defined differently in the different versions of pg_catalog. Note that search_path is set to ‘public, pg_catalog’ so I’m also confused why I have to set the schema.
Again I’m not a real PostgreSQL DBA so try to be kind.
Oof, those are really old postgres versions, including the version you're upgrading to (8.4 was released in 2009, and support ended in 2014).
The short answer is that, as long as upsinfo and data are being created and populated, you're probably fine, and good to go. But one of your foreign key relationships is broken.
The long answer, well, let me see if I can explain what is going on (or, at least, what I think is going on).
I'm guessing that the original table definition of data included something like FOREIGN KEY (ups) REFERENCES upsinfo (ups) ON DELETE CASCADE. That causes postgres to automatically make some trigger constraints: 1- every time there's a new row for data, make sure that its ups column matches an existing row in upsinfo, and 2- every time you delete a row from upsinfo, delete the corresponding rows in data, based on the matching ups value.
That (not very informative) error message can come up when the foreign key relationship doesn't work. In order for a foreign key to make sense, the referenced value needs to be unique -- there should be only one row in upsinfo for each distinct value of ups. In order for postgres to know that, there needs to be a unique index or primary key on upsinfo.ups.
In this case, one of a couple things could be breaking it:
There's no primary key or unique index on upsinfo.ups (postgres should not have allowed a foreign key, but may have in very old versions)
There used to be a unique index, but it hadn't properly enforced uniqueness, so it didn't get successfully imported (a bug, again likely from a very old version)
In either case, if that foreign key relationship is important, you can try to fix it once the import is complete. Start by trying to make a unique index on upsinfo.ups, and see if you have problems. If you do, resolve the duplicate entries, and try again till it works. Then issue something like:
ALTER TABLE data
ADD FOREIGN KEY (ups) REFERENCES upsinfo (ups) ON DELETE CASCADE;
Of course, if things are working, it's possible you don't need to fix the foreign key, in which case you're probably able to ignore those errors and just move forward.
Hope that helps, and good luck!
This seems to be a part of ON DELETE CONSTRAINT. If I were you I would delete all such statements and replace them with a proper constraint definition on the target table.
Table definition should then look like this:
CREATE TABLE bookings (
boo_id serial NOT NULL,
boo_hotelid character varying NOT NULL,
boo_roomid integer NOT NULL,
CONSTRAINT pk_bookings
PRIMARY KEY (boo_id),
CONSTRAINT fk_bookings_boo_roomid
FOREIGN KEY (boo_roomid)
REFERENCES rooms (roo_id) MATCH SIMPLE
ON UPDATE CASCADE ON DELETE CASCADE
) WITHOUT OIDS;
And this part is what will internally create the trigger:
CONSTRAINT fk_bookings_boo_roomid
FOREIGN KEY (boo_roomid)
REFERENCES rooms (roo_id) MATCH SIMPLE
ON UPDATE CASCADE ON DELETE CASCADE
But, to be honest, I do not have an understanding for an upgrade to an unsupported version. You know the Postgres is version 9.5 now, right?
I'm trying to add a unique constraint on 2 columns on DB2-- docType column and title column. The values of each column can repeat in themselves. however, the values of (docType, title) pairs should not repeat.
I tried so far
ALTER TABLE externalfiles
ADD CONSTRAINT logicalKey UNIQUE (doctype, title)
and
alter table externalfiles add unique (doctype, title)
, and got the following error to both:
Operation not allowed for reason code "7" on table "PIT.EXTERNALFILES".. SQLCODE=-668, SQLSTATE=57016, DRIVER=4.21.29
Isn't this any allowed??
DB2 Unique Constraint over multiple Columns suggests creating indexes as alternative to this. Haven't tried indexing them yet-- however, i'm wondering why the unique constraint isn't working.
TIA.
This web search on db2 luw sqlstate 57016 returns a number of links that suggest a prior ALTER may require that a REORG be performed before the ADD CONSTRAINT can be effected.
Perhaps most notable was the following doc link and snippet of text describing the RC7 [apparently SQLERRMC=7]:
DB2 for Linux UNIX and Windows 9.7.0
->Database fundamentals
->Messages
->SQL Messages
->SQL0500 - SQL0999
->SQL0668N
…
7 • The table is in the reorg pending state. This can occur after an
ALTER TABLE statement containing a REORG-recommended operation.
…
First try to reorganize table by
call sysproc.admin_cmd ('reorg table <TABLE_NAME>');
Than try again to create unique index. It should work.
HI !
This is my table:
CREATE TABLE [ORG].[MyTable](
..
[my_column2] UNIQUEIDENTIFIER NOT NULL CONSTRAINT FK_C1 REFERENCES ORG.MyTable2 (my_column2),
[my_column3] INT NOT NULL CONSTRAINT FK_C2 REFERENCES ORG.MyTable3 (my_column3)
..
)
I've written this constraint to assure that combination my_column2 and my_column3 is always unique.
ALTER TABLE [ORG].[MyTable] ADD
CONSTRAINT UQ_MyConstraint UNIQUE NONCLUSTERED
(
my_column2,
my_column3
)
But then suddenly.. The DB stopped responding.. there is a lock or something..
Do you have any idea why?
What is bad with the constraint?
What I've already checked is that I have some locked objects when I select * from master.dbo.syslockinfo (joined with sysprocesses). After 10 minutes of inactivity.. this list is empty again and I can access all the objects in database. Weird..
It has to lock the table while checking the data to see if it violates the constraint or not, otherwise some bad data might get inserted while it was doing this
Some operations like rebuilding an index (Of course not using the ONLINE in Enterprise Edition) also will make the table inaccessible while it does this