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Add item for changing a column's data type.

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Bruce Momjian 2003-05-26 19:25:20 +00:00
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doc/FAQ
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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Tue Apr 22 14:02:41 EDT 2003
Last updated: Mon May 26 15:25:04 EDT 2003
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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4.1) What is the difference between binary cursors and normal cursors?
4.2) How do I SELECT only the first few rows of a query?
4.3) How do I get a list of tables or other things I can see in psql?
4.4) How do you remove a column from a table?
4.4) How do you remove a column from a table, or change it's data
type?
4.5) What is the maximum size for a row, a table, and a database?
4.6) How much database disk space is required to store data from a
typical text file?
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the -E option so it will print out the queries it uses to execute the
commands you give.
4.4) How do you remove a column from a table?
4.4) How do you remove a column from a table, or change its data type?
This functionality was added in release 7.3 with ALTER TABLE DROP
COLUMN. In earlier versions, you can do this:
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
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ALTER TABLE new_table RENAME TO old_table;
COMMIT;
To change the data type of a column, do this:
BEGIN;
ALTER TABLE tab ADD COLUMN new_col new_data_type;
UPDATE tab SET new_col = CAST(old_col AS new_data_type);
ALTER TABLE DROP COLUMN old_col;
COMMIT;
You might then want to do VACUUM FULL tab to reclaim the disk space
used by the expired rows.
4.5) What is the maximum size for a row, a table, and a database?
These are the limits:

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doc/src/FAQ/FAQ.html
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alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Tue Apr 22 14:02:41 EDT 2003</P>
<P>Last updated: Mon May 26 15:25:04 EDT 2003</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
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<A href="#4.3">4.3</A>) How do I get a list of tables or other
things I can see in <I>psql</I>?<BR>
<A href="#4.4">4.4</A>) How do you remove a column from a
table?<BR>
table, or change it's data type?<BR>
<A href="#4.5">4.5</A>) What is the maximum size for a row, a
table, and a database?<BR>
<A href="#4.6">4.6</A>) How much database disk space is required
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execute the commands you give.</P>
<H4><A name="4.4">4.4</A>) How do you remove a column from a
table?</H4>
table, or change its data type?</H4>
<P>This functionality was added in release 7.3 with
<P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3 with
<SMALL>ALTER TABLE DROP COLUMN</SMALL>. In earlier versions,
you can do this:</P>
<PRE>
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COMMIT;
</PRE>
<P>To change the data type of a column, do this:</P>
<PRE>
BEGIN;
ALTER TABLE tab ADD COLUMN new_col <i>new_data_type</i>;
UPDATE tab SET new_col = CAST(old_col AS <i>new_data_type</i>);
ALTER TABLE DROP COLUMN old_col;
COMMIT;
</PRE>
<P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
disk space used by the expired rows.</P>
<H4><A name="4.5">4.5</A>) What is the maximum size for a row, a
table, and a database?</H4>