Re: Zedstore - compressed in-core columnar storage

On 09.04.2019 18:08, Heikki Linnakangas wrote:
> On 09/04/2019 18:00, Konstantin Knizhnik wrote:
>> On 09.04.2019 17:09, Konstantin Knizhnik wrote:
>>> standard Postgres heap and my VOPS extension.
>>> As test data I used TPC-H benchmark (actually only one lineitem table
>>> generated with tpch-dbgen utility with scale factor 10 (~8Gb database).
>>> I attached script which I have use to populate data (you have to to
>>> download, build and run tpch-dbgen yourself, also you can comment code
>>> related with VOPS).
>
> Cool, thanks!
>
>>> Unfortunately I failed to load data in zedstore:
>>>
>>> postgres=# insert into zedstore_lineitem_projection (select
>>> l_shipdate,l_quantity,l_extendedprice,l_discount,l_tax,l_returnflag::"char",l_linestatus::"char" 
>>>
>>> from lineitem);
>>> psql: ERROR:  compression failed. what now?
>>> Time: 237804.775 ms (03:57.805)
>
> Yeah, it's still early days, it will crash and burn in a lot of cases. 
> We wanted to publish this early, to gather ideas and comments on the 
> high level design, and to validate that the table AM API that's in v12 
> is usable.
>
>> Looks like the original problem was caused by internal postgres
>> compressor: I have not configured Postgres to use lz4.
>> When I configured Postgres --with-lz4, data was correctly inserted in
>> zedstore table, but looks it is not compressed at all:
>>
>> postgres=# select pg_relation_size('zedstore_lineitem_projection');
>>    pg_relation_size
>> ------------------
>>          9363010640
>
> The single-insert codepath isn't very optimized yet. If you populate 
> the table with large "INSERT ... SELECT ...", you end up with a huge 
> undo log. Try loading it with COPY.
>
> You can also see how many pages of each type there is with:
>
> select count(*), pg_zs_page_type('zedstore_lineitem_projection', g)
>   from generate_series(0, 
> pg_table_size('zedstore_lineitem_projection') / 8192 - 1) g group by 2;
>
> - Heikki

postgres=# copy zedstore_lineitem from '/mnt/data/lineitem.tbl' 
delimiter '|' csv;
COPY 59986052
Time: 232802.257 ms (03:52.802)
postgres=# select pg_relation_size('zedstore_lineitem');
  pg_relation_size
------------------
       10346504192
(1 row)
postgres=# select count(*), pg_zs_page_type('zedstore_lineitem', g)
   from generate_series(0, pg_table_size('zedstore_lineitem') / 8192 - 
1) g group by 2;
   count  | pg_zs_page_type
---------+-----------------
        1 | META
  1262308 | BTREE
      692 | UNDO
(3 rows)

And now performance is much worser:
Time: 99819.476 ms (01:39.819)

It is strange, because the main advantage of columnar store is that it 
has to fetch only accessed rows.
What I see is that in non-parallel mode (max_parallel_workers_per_gather 
= 0)
backend consumes about 11GB of memory. It fits in my desktop RAM (16GB) 
and speed is ~58 seconds.
But one I start 4 parallel workers, them cause huge swapping:

   PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ 
COMMAND
28195 knizhnik  20   0 11.823g 6.553g 5.072g D   7.6 42.2   0:17.19 
postgres
28074 knizhnik  20   0 11.848g 6.726g 5.223g D   7.3 43.3   4:14.96 
postgres
28192 knizhnik  20   0 11.854g 6.586g 5.075g D   7.3 42.4   0:17.18 
postgres
28193 knizhnik  20   0 11.870g 6.594g 5.064g D   7.3 42.4   0:17.19 
postgres
28194 knizhnik  20   0 11.854g 6.589g 5.078g D   7.3 42.4   0:17.09 
postgres

which is also strange because data should be present in shared buffers.

-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company