Re: How to speed up pg_trgm / gin index scan - Mailing list pgsql-general

On 22/06/15 13:51, Christian Ramseyer wrote:
> Hi
>
> I have a pretty large table with syslog messages.
>
> It is already partitioned by month, and for a single month I have e.g.
>
>
> DM=# \d+ logs_01
>
>     Column    |            Type             |
> --------------+-----------------------------+
>  host         | character varying(255)      |
>  facility     | character varying(10)       |
>  priority     | character varying(10)       |
>  tag          | character varying(255)      |
>  log_date     | timestamp without time zone |
>  program      | character varying(255)      |
>  msg          | text                        |
>  seq          | bigint                      |
>
> Indexes:
>     "logs_01_pkey" PRIMARY KEY, btree (seq)
>     "idx_logs_01_lower_host" btree (lower(host::text) varchar_pattern_ops)
>     "logs_01_date_index" btree (log_date)
>     "tridx_logs_01_msg" gin (msg gin_trgm_ops)
>
>
> DM=# select count(*) from logs_01;
>   count
> ----------
>  83052864
>
>
> I'd like to provide a fast "like %x%" search on the msg column, hence I added a trigram based gin index on it. It is around 60 GB on the 35 GB table:
>
> DM=# select count(*) from logs_01;
>   count
> ----------
>  83052864
>
>
> DM=# \dt+ logs_01
>                      List of relations
>  Schema |  Name   | Type  |  Owner   | Size  | Description
> --------+---------+-------+----------+-------+-------------
>  public | logs_01 | table | postgres | 35 GB |
>
> DM=# \di+ tridx_logs_01_msg
>                                List of relations
>  Schema |       Name        | Type  |  Owner   |  Table  | Size  | Description
> --------+-------------------+-------+----------+---------+-------+-------------
>  public | tridx_logs_01_msg | index | postgres | logs_01 | 58 GB |
>
>
> A typical query on this table looks like this:
>
> explain analyze
> select log_date, host, msg
> from logs_01 as log   where  log.msg like '%192.23.33.177%'
>     and log.log_date >= '2015-1-18 1:45:24'
>     and log.log_date <= '2015-1-19 1:45:24'
>     order by log_date asc offset 200 limit 50;
>
>
> It yields a promising explain that shows that the index is used, but unfortunately the bitmap index scan on the GIN index takes quite long (40 seconds)
>
>                                                                                                 QUERY PLAN
> ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>  Limit  (cost=34510.06..34510.06 rows=1 width=195) (actual time=42971.002..42971.015 rows=50 loops=1)
>    ->  Sort  (cost=34509.75..34510.06 rows=124 width=195) (actual time=42970.960..42970.990 rows=250 loops=1)
>          Sort Key: log_date
>          Sort Method: top-N heapsort  Memory: 152kB
>          ->  Bitmap Heap Scan on logs_01 log  (cost=34009.21..34505.44 rows=124 width=195) (actual time=42963.969..42969.725 rows=2472 loops=1)
>                Recheck Cond: ((msg ~~ '%192.23.33.177%'::text) AND (log_date >= '2015-01-18 01:45:24'::timestamp without time zone) AND (log_date <= '2015-01-19 01:45:24'::timestamp without time zone))
>                ->  BitmapAnd  (cost=34009.21..34009.21 rows=124 width=0) (actual time=42962.562..42962.562 rows=0 loops=1)
>                      ->  Bitmap Index Scan on tridx_logs_01_msg  (cost=0.00..6992.15 rows=8020 width=0) (actual time=42731.145..42731.145 rows=168489 loops=1)
>                            Index Cond: (msg ~~ '%192.23.33.177%'::text)
>                      ->  Bitmap Index Scan on logs_01_date_index  (cost=0.00..27016.75 rows=1287939 width=0) (actual time=180.055..180.055 rows=1173048 loops=1)
>                            Index Cond: ((log_date >= '2015-01-18 01:45:24'::timestamp without time zone) AND (log_date <= '2015-01-19 01:45:24'::timestamp without time zone))
>  Total runtime: 42971.137 ms
>
> (also on http://explain.depesz.com/s/KpaB)
>
>
> Any good ideas on how I could speed this up a bit?
>
> I have already tried to throw quite a bunch of memory at the problem:
>
> shared_buffers = 64GB
> work_mem = 16GB
>
> but it didn't improve between this and the 32GB shared/ 2GB work GB I had before.
>
> This is on Postgres 9.1.15 on Linux.
>

> Try 9.4 and you'll surprise.
>
> 1. GIN has compression
> 2. GIN has fast scan feature.
>
> Oleg


Hi Oleg and List

I finally got around to try 9.4, and it is quite fantastic.

Index size went from 58 to now 14 GB:

DM=# \di+ tridx_logs_01_msg
                               List of relations
 Schema |       Name        | Type  |  Owner   |  Table  | Size  |
Description
--------+-------------------+-------+----------+---------+-------+-------------
 public | tridx_logs_01_msg | index | postgres | logs_01 | 14 GB |


And the time for the above query went down to about 20 seconds:


DM=# explain analyze
DM-# select log_date, host, msg
DM-# from logs_01 as log   where  log.msg like '%192.23.33.177%'
DM-#     and log.log_date >= '2015-1-18 1:45:24'
DM-#     and log.log_date <= '2015-1-19 1:45:24'
DM-#     order by log_date asc offset 200 limit 50;

                        QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=28815.06..28815.06 rows=1 width=194) (actual
time=19032.099..19032.099 rows=0 loops=1)
   ->  Sort  (cost=28814.74..28815.06 rows=128 width=194) (actual
time=19032.093..19032.093 rows=0 loops=1)
         Sort Key: log_date
         Sort Method: quicksort  Memory: 25kB
         ->  Bitmap Heap Scan on logs_01 log  (cost=28298.06..28810.26
rows=128 width=194) (actual time=19031.992..19031.992 rows=0 loops=1)
               Recheck Cond: ((msg ~~ '%192.23.33.177%'::text) AND
(log_date >= '2015-01-18 01:45:24'::timestamp without time zone) AND
(log_date <= '2015-01-19 01:45:24'::timestamp without time zone))
               ->  BitmapAnd  (cost=28298.06..28298.06 rows=128 width=0)
(actual time=19031.983..19031.983 rows=0 loops=1)
                     ->  Bitmap Index Scan on tridx_logs_01_msg
(cost=0.00..508.15 rows=8020 width=0) (actual time=18408.121..18408.121
rows=99 loops=1)
                           Index Cond: (msg ~~ '%192.23.33.177%'::text)
                     ->  Bitmap Index Scan on logs_01_date_index
(cost=0.00..27789.60 rows=1325303 width=0) (actual time=623.084..623.084
rows=1173048 loops=1)
                           Index Cond: ((log_date >= '2015-01-18
01:45:24'::timestamp without time zone) AND (log_date <= '2015-01-19
01:45:24'::timestamp without time zone))
 Planning time: 0.945 ms
 Execution time: 19032.409 ms
(13 rows)

Great stuff! Sorry Oleg I don't have your original message anymore and
can't reply into the right place in the thread, so I took the liberty to
CC: you.

Christian





--
Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general