RE: select query does not pick up the right index - Mailing list pgsql-performance

Lana ABADIE
Database Engineer
CODAC Section

ITER Organization, Building 72/4108, SCOD, Control System Division
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-----Original Message-----
From: David Rowley <david.rowley@2ndquadrant.com> 
Sent: 03 January 2019 14:18
To: Abadie Lana <Lana.Abadie@iter.org>
Cc: pgsql-performance@lists.postgresql.org
Subject: Re: select query does not pick up the right index

> From: David Rowley <david.rowley@2ndquadrant.com>
> Sent: 03 January 2019 14:01
> That's surprisingly slow. Can you share the EXPLAIN (ANALYZE, BUFFERS) of that?
>
> explain (analyze,buffers) select 
> 'BUIL-B36-VA-RT-RT1:CL0001-2-ABW',c.smpl_time,c.nanosecs,c.float_val,c
> .num_val,c.str_val,c.datatype,c.array_val from sample c WHERE 
> c.channel_id = (SELECT channel_id FROM channel WHERE 
> name='BUIL-B36-VA-RT-RT1:CL0001-2-ABW') order by c.smpl_time desc 
> limit 5;


>                ->  Index Scan Backward using smpl_time_bx2_idx on 
> sample_buil_year c_5  (cost=0.56..2023925.30 rows=3162364
> width=320) (actual time=15167.330..15167.330 rows=0 loops=1)
>                      Filter: (channel_id = $0)
>                      Rows Removed by Filter: 50597834
>                      Buffers: shared hit=25913147 read=713221
>                ->  Index Scan Backward using sample_time_cm_idx on 
> sample_ctrl_month c_6  (cost=0.56..1862587.12 rows=537562
>  width=77) (actual time=0.048..0.048 rows=0 loops=1)
>                      Index Cond: (channel_id = $0)
>                      Buffers: shared read=4
>                ->  Index Scan Backward using smpl_time_cmx2_idx on 
> sample_ctrl_year c_7  (cost=0.57..3186305.67 rows=2094186
>  width=68) (actual time=25847.549..25847.549 rows=0 loops=1)
>                      Filter: (channel_id = $0)
>                      Rows Removed by Filter: 79579075
>                      Buffers: shared hit=49868991 read=1121715

Right, so you need to check your indexes on sample_ctrl_year and sample_buil_year. You need an index on (channel_id,
smpl_time)on those.
 


These indexes exist already
\d sample_ctrl_year
                        Table "public.sample_ctrl_year"
   Column    |            Type             | Collation | Nullable |   Default
-------------+-----------------------------+-----------+----------+-------------
 channel_id  | bigint                      |           | not null |
 smpl_time   | timestamp without time zone |           | not null |
 nanosecs    | bigint                      |           | not null |
 severity_id | bigint                      |           | not null |
 status_id   | bigint                      |           | not null |
 num_val     | integer                     |           |          |
 float_val   | double precision            |           |          |
 str_val     | character varying(120)      |           |          |
 datatype    | character(1)                |           |          | ' '::bpchar
 array_val   | bytea                       |           |          |
Indexes:
    "sample_time_cy_idx" btree (channel_id, smpl_time)
    "sample_time_yc1_idx" btree (smpl_time, channel_id)
    "smpl_time_cmx2_idx" btree (smpl_time)
Check constraints:
    "sample_ctrl_year_smpl_time_check" CHECK (smpl_time >= (now() - '1 year 1 mon'::interval)::timestamp without time
zoneAND smpl_time <= now())
 
Inherits: sample_ctrl

css_archive_3_0_0=# \d sample_buil_year
                        Table "public.sample_buil_year"
   Column    |            Type             | Collation | Nullable |   Default
-------------+-----------------------------+-----------+----------+-------------
 channel_id  | bigint                      |           | not null |
 smpl_time   | timestamp without time zone |           | not null |
 nanosecs    | bigint                      |           | not null |
 severity_id | bigint                      |           | not null |
 status_id   | bigint                      |           | not null |
 num_val     | integer                     |           |          |
 float_val   | double precision            |           |          |
 str_val     | character varying(120)      |           |          |
 datatype    | character(1)                |           |          | ' '::bpchar
 array_val   | bytea                       |           |          |
Indexes:
    "sample_time_by_idx" btree (channel_id, smpl_time)
    "sample_time_yb1_idx" btree (smpl_time, channel_id)
    "smpl_time_bx2_idx" btree (smpl_time)
Check constraints:
    "sample_buil_year_smpl_time_check" CHECK (smpl_time >= (now() - '1 year 1 mon'::interval)::timestamp without time
zoneAND smpl_time <= now())
 
Inherits: sample_buil

css_archive_3_0_0=#

-- 
 David Rowley                   http://www.2ndQuadrant.com/
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