Frequently Asked Questions (FAQ) for PostgreSQL
- Last updated: Mon Jan 31 20:35:58 EST 2005
+ Last updated: Mon Jan 31 20:41:21 EST 2005
4.1) How do I SELECT only the first few rows of a query? A random row?
4.2) How do I find out what tables, indexes, databases, and users are
defined? How do I see the queries used by psql to display them?
- 4.3) How do you remove a column from a table, or change its data type?
+ 4.3) How do you change a column's data type?
4.4) What is the maximum size for a row, a table, and a database?
4.5) How much database disk space is required to store data from a
typical text file?
query to get information about the database.
There are also system tables beginning with pg_ that describe these
- too. Use psql -l will list all databases.
+ too.
+
+ Use psql -l will list all databases.
Also try the file pgsql/src/tutorial/syscat.source. It illustrates
many of the SELECTs needed to get information from the database system
tables.
- 4.3) How do you remove a column from a table, or change its data type?
+ 4.3) How do you change a column's data type?
- DROP COLUMN functionality was added in release 7.3 with ALTER TABLE
- DROP COLUMN. In earlier versions, you can do this:
- BEGIN;
- LOCK TABLE old_table;
- SELECT ... -- select all columns but the one you want to remove
- INTO TABLE new_table
- FROM old_table;
- DROP TABLE old_table;
- ALTER TABLE new_table RENAME TO old_table;
- COMMIT;
-
Changing the data type of a column can be done easily in 8.0 and later
with ALTER TABLE ALTER COLUMN TYPE.