The following bug has been logged online:

Bug reference: 2450
Logged by: Anjali
Email address: anjalia (AT) cdac (DOT) in
PostgreSQL version: 7.4.8
Operating system: fedora core3
Description: change the type of existing column
Details:

i am getting difficulty to change the type of the existing table in
postgresql.
is this version of postgresql supported change of data type of the existing
column.
plz reply me as soon as possible.

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TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match

Anjali wrote:
ALTER TABLE ALTER COLUMN TYPE was added in 8.0.

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Bruce Momjian http://candle.pha.pa.us
EnterpriseDB http://www.enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +

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TIP 3: Have you checked our extensive FAQ?

http://www.postgresql.org/docs/faq