Re: [HACKERS] Custom compression methods

Hi,

On 11/23/2017 10:38 AM, Ildus Kurbangaliev wrote:
> On Tue, 21 Nov 2017 18:47:49 +0100
> Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>
>>>
>>
>> Hmmm, it still doesn't work for me. See this:
>>
>> test=# create extension pg_lz4 ;
>> CREATE EXTENSION
>> test=# create table t_lz4 (v text compressed lz4);
>> CREATE TABLE
>> test=# create table t_pglz (v text);
>> CREATE TABLE
>> test=# insert into t_lz4 select repeat(md5(1::text),300);
>> INSERT 0 1
>> test=# insert into t_pglz select * from t_lz4;
>> INSERT 0 1
>> test=# drop extension pg_lz4 cascade;
>> NOTICE: drop cascades to 2 other objects
>> DETAIL: drop cascades to compression options for lz4
>> drop cascades to table t_lz4 column v
>> DROP EXTENSION
>> test=# \c test
>> You are now connected to database "test" as user "user".
>> test=# insert into t_lz4 select repeat(md5(1::text),300);^C
>> test=# select * from t_pglz ;
>> ERROR: cache lookup failed for compression options 16419
>>
>> That suggests no recompression happened.
>
> Should be fixed in the attached patch. I've changed your extension a
> little bit according changes in the new patch (also in attachments).
>

Hmm, this seems to have fixed it, but only in one direction. Consider this:

create table t_pglz (v text);
create table t_lz4 (v text compressed lz4);

insert into t_pglz select repeat(md5(i::text),300)
from generate_series(1,100000) s(i);

insert into t_lz4 select repeat(md5(i::text),300)
from generate_series(1,100000) s(i);

\d+

Schema | Name | Type | Owner | Size | Description
--------+--------+-------+-------+-------+-------------
public | t_lz4 | table | user | 12 MB |
public | t_pglz | table | user | 18 MB |
(2 rows)

truncate t_pglz;
insert into t_pglz select * from t_lz4;

\d+

Schema | Name | Type | Owner | Size | Description
--------+--------+-------+-------+-------+-------------
public | t_lz4 | table | user | 12 MB |
public | t_pglz | table | user | 18 MB |
(2 rows)

which is fine. But in the other direction, this happens

truncate t_lz4;
insert into t_lz4 select * from t_pglz;

\d+
List of relations
Schema | Name | Type | Owner | Size | Description
--------+--------+-------+-------+-------+-------------
public | t_lz4 | table | user | 18 MB |
public | t_pglz | table | user | 18 MB |
(2 rows)

which means the data is still pglz-compressed. That's rather strange, I
guess, and it should compress the data using the compression method set
for the target table instead.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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