Server does not reply to Alter Table

Using pgsql 7.1.3 I have found that on occasions, there is no reply to an
SQL statement like

ALTER TABLE x
    ADD COLUMN y z

The table itself will be inaccessible for at least 10 minutes and/or error
messages will result. The column may or may not be added.

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Patrick Dunford, Christchurch, NZ
http://pdunford.godzone.net.nz/
http://www.trainweb.org/ferrymead/