I have a query that it seems is destined to be slow one way or another. I
have a table of around 30k articles, categorized by topic and ordered by date:
create table articles (
topic varchar(50),
created date,
data text
);
create index articles_topic_idx on articles(topic);
create index articles_created_idx on articles(created);
If I want to get the 5 most recent articles in a topic, I get a nice query
plan and a fast query:
# explain select * from articles where topic = 'Example' order by created desc
limit 5 ;
QUERY PLAN
-------------------------------------------------------------------------------
-----------------------------
Limit (cost=0.00..646.71 rows=5 width=828)
-> Index Scan Backward using articles_created_idx on articles
(cost=0.00..85202.16 rows=659 width=828)
Filter: (topic = 'Example'::character varying)
(3 rows)
# select * from articles where topic = 'Example' order by created desc limit 5
;
[.....]
Time: 18.42 ms
However, if the topic happens to not exist, this query takes a very long time:
# select * from articles where topic = 'NO-Example' order by created desc
limit 5 ;
[.....]
Time: 1075.36 ms
If I drop the date index or get more articles (so it doesn't do the backward
scan on articles_created_idx), then the situation is reversed: getting the
most recent articles for a topic that exists takes a fair amount of time,
while getting a topic that does not exist is nearly instantaneous.
Is there any way I can get the best of both worlds?
-J
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