Re: Force re-compression with lz4

On 10/17/21 11:36 AM, Ron wrote:
> 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;
I forgot to mention:
VACUUM t;
> COPY t FROM '/some/file.csv';

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