ads' corner - SQL

role ... cannot be dropped because some objects depend on it

nospam@example.com (Andreas 'ads' Scherbaum) — Wed, 10 Sep 2008 20:11:07 +0200

Andreas 'ads' Scherbaum

I want to drop a ROLE (previously USER/GROUPs) which is still referenced by object(s) in my database. But PostgreSQL has objections:

ERROR: role "xyz" cannot be dropped because some objects depend on it

DETAIL: access to schema public

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create language if not exist

nospam@example.com (Andreas 'ads' Scherbaum) — Thu, 27 Mar 2008 08:33:14 +0100

Andreas 'ads' Scherbaum

In a customer project i have to setup a database from a Makefile. Part of my problem: on windows the installer may or may not install "plpgsql" into template1 so in consequence this language would be activated in every new database. But that's not predictable. This problem can appear on different Linux/Unix distributions too.

In contrary to some other opinions in #postgresql i dislike filtering error messages and i prefer clean solutions.

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Understanding SQL

nospam@example.com (Andreas 'ads' Scherbaum) — Sun, 13 Jan 2008 15:29:56 +0100

Andreas 'ads' Scherbaum

Just have to port a phpkit database to something else. It seems, the phpkit ppl did not really understand what SQL is for.

The 'phpkit_config' table in this case contains 255(!) columns and a lot of this columns seems to be for extensions.

This is a very good example where you should use normalization: you don't add more and more columns to one table but you have a key/value table and for new entries you add a new key with a value. No need to change the table at all.

table size, database size

nospam@example.com (Andreas 'ads' Scherbaum) — Tue, 04 Dec 2007 11:07:00 +0100

Andreas 'ads' Scherbaum

I have seen this question more than once in the past: "How much storage space do i need for my table?"
PostgreSQL can give you this information.

The storage space (in bytes) for one specific table:

ads=# select pg_relation_size('size_test');
pg_relation_size
------------------
5668864
(1 row)

Same query, but with the result in a human-readable format:

ads=# select pg_size_pretty(pg_relation_size('size_test'));
pg_size_pretty
----------------
5536 kB(1 row)

Remember that this is only the size of the table, not included an index or additional stuff.
The size including any index can be found out with:

ads=# select pg_size_pretty(pg_total_relation_size('size_test'));
pg_size_pretty
----------------
7656 kB
(1 row)

The size of a complete database:

ads=# select pg_size_pretty(pg_database_size('ads'));
pg_size_pretty
----------------
11 MB
(1 row)

You can even find out, how much space a specific value needs:

ads=# select pg_column_size(5::smallint);
pg_column_size
----------------
2
(1 row)

ads=# select pg_column_size(5::int);
pg_column_size
----------------
4
(1 row)

ads=# select pg_column_size(5::bigint);
pg_column_size
----------------
8
(1 row)

ads=# select pg_column_size('This is a string'::varchar);
pg_column_size
----------------
20
(1 row)

ads=# select length('This is a string'::varchar);
length
--------
16
(1 row)

The 4 byte difference between the column size and the string length is the storage overhead needed for varchar columns.

The size for a tablespace can be found out with pg_tablespace_size(), but i don't have a machine with configured tablespace at hand to show an example.