How to delete unused sequences?

问题:

We are using PostgreSQL. My requirement is to delete unused sequences from my database. For example, if I create any table through my application, one sequence will be created, but for deleting the table we are not deleting the sequence, too. If want to create the same table another sequence is being created.

Example: table: file; automatically created sequence for id coumn: file_id_seq

When I delete the table file and create it with same name again, a new sequence is being created (i.e. file_id_seq1). I have accumulated a huge number of unused sequences in my application database this way.

How to delete these unused sequences?

回答1:

First off, a sequence that is created automatically for a serial column is deleted automatically, when the column (or table it is in) is deleted. The problem you describe should not exist to begin with. Only very old versions of PostgreSQL did not do that. 7.4 or older?

Solution for the problem:

This query will generate the DDL commands to delete all "unbound" sequences in the database it is executed in:

SELECT string_agg('DROP SEQUENCE ' || c.oid::regclass, '; ') || ';' AS ddl
FROM   pg_class       c
LEFT   JOIN pg_depend d ON d.refobjid = c.oid
                       AND d.deptype <> 'i'
WHERE  c.relkind = 'S'
AND    d.refobjid IS NULL;

The cast to regclass in c.oid::regclass automatically schema-qualifies sequence names where necessary according to the current search_path. See:

• How to check if a table exists in a given schema
• How does the search_path influence identifier resolution and the "current schema"

Result:

DROP SEQUENCE foo_id_seq;
DROP SEQUENCE bar_id_seq;
...

Execute the result to drop all sequences that are not bound to a serial column (or any other column). Study the meaning of columns and tables here.

Careful though! It does not mean those sequences aren't in use otherwise. There are a number of use cases where sequences are created as standalone objects. For instance if you want multiple columns to share one sequence. You should know exactly what you are doing.

However, you cannot delete sequences bound to a serial column this way. So the operation is safe in this respect.

DROP SEQUENCE test_id_seq

Result:

ERROR:  cannot drop sequence test_id_seq because other objects depend on it
DETAIL:  default for table test column id depends on sequence test_id_seq
HINT:  Use DROP ... CASCADE to drop the dependent objects too.

回答2:

If you are using pgAdmin, you can select the sequence and check the "depends on" tab. It will list any object that relies on the sequence.

Another way is to TRY to delete the sequence. If a table references it, pgAdmin will throw an error saying that something is depending on this sequence. If you are able to delete the sequence without any errors, there is no dependency.

Be sure to test this somewhere.

回答3:

What i do is first I got all the sequences and then saved these result into a file then i run the file in psql: below content was saved with file name del_seq_all.sql and then list sequences in test1.sql . i dont know this is the correct solution or not. But result is coming as expected.

\o d:/test1.sql
SELECT 'drop sequence ' || c.relname || ';' FROM pg_class c WHERE
(c.relkind = 'S');
\o

\i d:/test1.sql

回答4:

Proceed with caution, "drop sequence sequence_name_here" will successfully drop a sequence even if it's attached as a default nextval() value of a table column. There seems to be some disconnect here especially if the sequence was created separately. I'm also looking for the perfect one liner to clean up 100% unused sequences.