the cost decreased but the time was practically the same, why? - postgresql

Question

Table definition:
CREATE TABLE schema.mylogoperation (
id_mylogoperation serial,
data DATE,
myschema VARCHAR(255),
column_var_2 VARCHAR(255),
user VARCHAR(255),
action TEXT,
column_var_1 TEXT,
log_old VARCHAR,
log_new VARCHAR
constraint pk_mylogoperation primary key (id_mylogoperation)
)
WITH (oids = false);
12 million rows
I tried to explain analyze:
explain analyze
SELECT
column_var_1,
column_var_2
column_var_3,
user,
action,
data,
log_old,
log_new
FROM schema.mylogoperation
WHERE
myschema = 'schema'
AND column_var_2 IN ('mydata1', 'mydata2', 'mydata3')
AND log_old <> log_new
AND column_var_1 LIKE 'mydata%';
indexes ( pk_mylogoperation only)
QUERY PLAN
Seq Scan on myschema (cost=0.00..713948.14rows=660 width=222) (actual time=380.308..4467.364 rows=48 loops=1)
Filter: (((log_old)::text <> (log_new)::text) AND (column_var_1 ~~ 'mydata%'::text) AND ((schema)::text = 'schema'::text) AND ((column_var_2)::text = ANY ('{mydata1,mydata2,mydata3}'::text[])))
Rows Removed by Filter: 12525296
Total runtime: 4467.425 ms
Then I tried to create a some index for better performance:
CREATE INDEX idx_mylogoperation_1 ON schema.mylogoperation (myschema, column_var_2);
reindex table schema.mylogoperation;
analyze schema.mylogoperation;
pk_mylogoperation + idx_mylogoperation_1
QUERY PLAN
Index Scan using idx_mylogoperation_qry1 on mylogoperation (cost=0.56..589836.84 rows=658 width=223) (actual time=331.679..4997.507 rows=48 loops=1)
Index Cond: (((myschema)::text = 'schema'::text) AND ((column_var_2)::text = ANY ('{mydata1,mydata2,mydata3}'::text[])))
Filter: (((log_old)::text <> (log_new)::text) AND (column_var_1 ~~ 'mydata%'::text))
Rows Removed by Filter: 7441986
Total runtime: 4997.580 ms
Then I tried again to create a some index for better performance:
CREATE INDEX idx_mylogoperation_2 ON schema.mylogoperation USING gin (column_var_1 gin_trgm_ops);
reindex table schema.mylogoperation;
analyze schema.mylogoperation;
pk_mylogoperation + idx_mylogoperation_1 + idx_mylogoperation_2
QUERY PLAN
Bitmap Heap Scan on idx_mylogoperation_var_1 (cost=1398.58..2765.08 rows=663 width=222) (actual time=5303.481..5303.906 rows=48 loops=1)
Recheck Cond: (column_var_1 ~~ 'mydata%'::text)
Filter: (((log_old)::text <> (log_new)::text) AND ((myschema)::text = 'schema'::text) AND ((column_var_2)::text = ANY ('{mydata1,mydata2,mydata3}'::text[])))
Rows Removed by Filter: 248
-> Bitmap Index Scan on idx_mylogoperation_var_1 (cost=0.00..1398.41 rows=1215 width=0) (actual time=5303.203..5303.203 rows=296 loops=1)
Index Cond: (column_var_1 ~~ 'mydata%'::text)
Total runtime: 5303.950 ms
The question
the cost decreased but the time was practically the same, why?
Notes:
I do not want to make changes to the select operation, just in the database structure.
This test was performed on a server that is in use. But creating these indices was efficient? Or rather do not use them.
I am using Postgres 9.3.22 on Linux 64-bit Red Hat.

Answer 1

This index:
CREATE INDEX idx_mylogoperation_1 ON schema.mylogoperation (myschema, column_var_2);
didn't help because the relevant portion of your where clause matched ~2/3 of the table. The index didn't narrow down the results very much, but the filter did:
Filter: (((log_old)::text <> (log_new)::text) AND (column_var_1 ~~ 'mydata%'::text))
Rows Removed by Filter: 7441986
I'm not sure which of those two things in the filter removed more, but you could try a partial index like:
CREATE INDEX idx_mylogoperation_1 ON schema.mylogoperation (myschema, column_var_2) WHERE log_old <> log_new;