Re: Force re-compression with lz4

On 10/17/21 10:12 AM, Florents Tselai wrote:
> Hello,
>
> I have a table storing mostly text data (40M+ rows) that has
> pg_total_relation_size ~670GB.
> I’ve just upgraded to postgres 14 and I’m now eager to try the new LZ4
> compression.
>
> I’ve altered the column to use the new lz4 compression, but that only
> applies to new rows.
>
> What’s the recommended way of triggering the re-evaluation for
> pre-existing rows?
>
> I tried wrapping a function like the following, but apparently each old
> record retains the compression applied.
> text_corpus=(SELECT t.text from...); delete from t where id=; insert into
> t(id, text) values (id, text_corpus);

Because it's all in one transaction?

> Fttb, I resorted to preparing an external shell script to execute against
> the db but that’s too slow as it moves data in&out the db.
>
> Is there a smarter way to do this ?

Even with in-place compression, you've got to read the uncompressed data.

Does your shell script process one record at a time?  Maybe do ranges:
COPY (SELECT * FROM t WHERE id BETWEEN x AND y) TO '/some/file.csv';
DELETE FROM t WHERE id BETWEEN x AND y;
COPY t FROM '/some/file.csv';

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