Hi everyone
I wrote a query to iterate over two different columns. These columns have a
composite unique index on them, and I’m only retrieving those columns. I found
that there was a very large difference in performance when using row wise
comparison versus a column comparison. The column comparison was much faster
and the row wise comparison showed an increase in both CPU and shared buffer
hits.
My expectation is that the following two queries would have roughly the same
performance. They both use the same index only scans and return the same 100
rows of data. The main difference I see in the explain output is that the row
wise comparison has 3,000 times the shared buffer hits, but it is unclear why
there would need to be more hits.
I’ve also found that when I add a limit 100 clause to the row wise comparison I
get roughly equivalent performance, but with limit 101, the extra shared hits
return, which leads me to believe that the <= condition is not stopping the
execution of the query.
We are running on AWS Aurora with version 16 and db.r5.24xlarge instance size.
I’ve attached 3 explains:
1. Well performing query without a row wise comparison
2. Poor performing query with row wise comparison
3. Well performing query with row wise comparison and limit
explain (analyze, buffers)
select data_model_id, primary_key
from entity
WHERE data_model_id = 123 AND primary_key >= ‘ABC’ AND primary_key <= ‘DEF’;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Index Only Scan using entity_data_model_id_primary_key_uniq on entity
(cost=0.70..2.92 rows=1 width=31) (actual time=0.088..0.154 rows=100 loops=1)
Index Cond: ((data_model_id = 287) AND (primary_key >= ‘ABC’::text) AND
(primary_key <= ‘DEF’::text))
Heap Fetches: 21
Buffers: shared hit=37
Planning Time: 0.111 ms
Execution Time: 0.175 ms
(6 rows)
explain (analyze, buffers)
select data_model_id, primary_key
from entity
WHERE (data_model_id, primary_key) >= (123 , ‘ABC’) AND (data_model_id,
primary_key) <= (123, ‘DEF’);
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Index Only Scan using entity_data_model_id_primary_key_uniq on entity
(cost=0.70..7135372.50 rows=15347234 width=31) (actual time=0.116..2779.874
rows=100 loops=1)
Index Cond: ((ROW(data_model_id, primary_key) >= ROW(123, ‘ABC’::text)) AND
(ROW(data_model_id, primary_key) <= ROW(123, ‘DEF’::text)))
Heap Fetches: 21
Buffers: shared hit=97261
Planning:
Buffers: shared hit=112
Planning Time: 0.324 ms
Execution Time: 2779.915 ms
(8 rows)
explain (analyze, buffers)
select data_model_id, primary_key
from entity
WHERE (data_model_id, primary_key) BETWEEN (123, ‘DEF’) AND (123, 'DEF') limit
100;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.70..6.31 rows=100 width=31) (actual time=0.075..0.175 rows=100
loops=1)
Buffers: shared hit=34
-> Index Only Scan using entity_data_model_id_primary_key_uniq on entity
(cost=0.70..873753.60 rows=15581254 width=31) (actual time=0.075..0.167
rows=100 loops=1)
Index Cond: ((ROW(data_model_id, primary_key) >= ROW(123,
'DEF'::text)) AND (ROW(data_model_id, primary_key) <= ROW(123, 'DEF'::text)))
Heap Fetches: 4
Buffers: shared hit=34
Planning:
Buffers: shared hit=104
Planning Time: 0.264 ms
Execution Time: 0.198 ms
(10 rows)
A truncated schema of our table:
Table "public.entity"
Column | Type | Collation | Nullable
| Default
------------------------------+--------------------------+-----------+----------+----------------------------------------
id | bigint | | not null
| nextval('api_entity_id_seq'::regclass)
updated_at | timestamp with time zone | | not null
|
created_at | timestamp with time zone | | not null
|
entity_id | uuid | | not null
|
primary_key | character varying(255) | | not null
|
data_model_id | bigint | | |
Indexes:
"api_entity_pkey" PRIMARY KEY, btree (id)
"entity_data_model_id_primary_key_uniq" UNIQUE, btree (data_model_id,
primary_key)
"api_entity_entity_id_key" UNIQUE CONSTRAINT, btree (entity_id)
"api_entity_updated_045756_idx" btree (updated_at DESC)
Check constraints:
"api_entity_data_model_v3_not_null" CHECK (data_model_v3_id IS NOT NULL)
Best,
Jon
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