I have a table with about 200.000 entries. Among other things, it
contains an integer field I use as a timestamp, and a variable
character field I use for user names. Certain queries are taking too
long IMO. I'm trying this on both 7.4 and 8.0.
If I do a direct comparison (using =) on the user name field and I
sort by the numeric field, I get about 5 ms. If I do a LIKE on the
user name and I don't sort at all, I get about 5 ms too. But if I use
both LIKE on the user name and sorting on the timestamp, the time
jumps to 2000 ms.
I have indexes on both fields, but I remember reading only one of them
will be used when doing a query.
Is such a serious jump in query times normal or am I doing something wrong?
I'm attaching the explain output for all 3 cases.
explain analyze select * from log_entries where user_name='john_doe' order by
timestamp desc limit 100 offset 0;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.00..235.47 rows=100 width=175) (actual time=0.945..5.858
rows=100 loops=1)
-> Index Scan Backward using timestamp_log_entries_key on log_entries
(cost=0.00..39093.47 rows=16602 width=175) (actual time=0.938..5.622 rows=100
loops=1)
Filter: ((user_name)::text = 'john_doe'::text)
Total runtime: 6.175 ms
(4 rows)
explain analyze select * from log_entries where user_name like '%john_doe%'
limit 100 offset 0;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------
Limit (cost=0.00..8250.17 rows=1 width=175) (actual time=0.495..3.364
rows=100 loops=1)
-> Seq Scan on log_entries (cost=0.00..8250.17 rows=1 width=175) (actual
time=0.486..3.138 rows=100 loops=1)
Filter: ((user_name)::text ~~ '%john_doe%'::text)
Total runtime: 3.657 ms
(4 rows)
explain analyze select * from log_entries where user_name like '%john_doe%'
order by timestamp desc limit 100 offset 0;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
Limit (cost=8250.18..8250.19 rows=1 width=175) (actual
time=1880.358..1880.910 rows=100 loops=1)
-> Sort (cost=8250.18..8250.19 rows=1 width=175) (actual
time=1880.345..1880.701 rows=100 loops=1)
Sort Key: "timestamp"
-> Seq Scan on log_entries (cost=0.00..8250.17 rows=1 width=175)
(actual time=0.432..1051.036 rows=15481 loops=1)
Filter: ((user_name)::text ~~ '%john_doe%'::text)
Total runtime: 1887.071 ms
(6 rows)
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