RE: select query does not pick up the right index

Lana ABADIE
Database Engineer
CODAC Section

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-----Original Message-----
From: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
Sent: 03 January 2019 14:18
To: Abadie Lana <Lana(dot)Abadie(at)iter(dot)org>
Cc: pgsql-performance(at)lists(dot)postgresql(dot)org
Subject: Re: select query does not pick up the right index

> From: David Rowley <david(dot)rowley(at)2ndquadrant(dot)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 zone AND 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 zone AND smpl_time <= now())
Inherits: sample_buil

css_archive_3_0_0=#

--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services