Re: How to speed up pg_trgm / gin index scan

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