pg_dump and truncate

Hello there,

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I was trying to do a live db archive using the following steps:

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1.    Open a connection to the database;
2.    With the connection, do the following:

BEGIN;

SET transaction ISOLATION level SERIALIZABLE;

3.    Execute the following from the Linux shell:

pg_dump dbname | gzip -9 > dbname-archive.gz

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4.    With the same connection, do the following:

truncate table1, table2,...;

end;

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Well, the tables are truncated all right, but the db dump file is
empty!!!

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What went wrong with my logic? What should be the correct procedure?


Thank you!


Feng