How to understand an EXPLAIN ANALYZE

Question

I am not very familiar with looking at EXPLAIN ANALYZE results, I have a huge problem with my queries being too slow. I have tried to read up on how to interpret results from an explain queries, but I still don't know what I should be looking for, and what might be wrong. I have a feeling that there is some big red light flashing somewhere, I just don't see it.

So the query is pretty simple, it looks like this:

EXPLAIN ANALYZE SELECT "cars".* FROM "cars" WHERE "cars"."sales_state" = 'onsale' AND "cars"."brand" = 'BMW' AND "cars"."model_name" = '318i' AND "cars"."has_auto_gear" = TRUE  LIMIT 25 OFFSET 0

And the result like this:

Limit  (cost=0.00..161.07 rows=25 width=1245) (actual time=35.232..38.694 rows=25 loops=1)
  ->  Index Scan using index_cars_onsale_on_brand_and_model_name on cars  (cost=0.00..1179.06 rows=183 width=1245) (actual time=35.228..38.652 rows=25 loops=1)
        Index Cond: (((brand)::text = 'BMW'::text) AND ((model_name)::text = '318i'::text))
        Filter: has_auto_gear"
Total runtime: 38.845 ms

A little background: I'm on Postgresql 9.1.6, running on Herokus dedicated databases. My db has aprox 7,5Gb RAM, the table cars contains 3,1M rows and an aprox 2,0M of the rows has sales_state = 'onsale'. The table has 170 columns. The index that it uses looks something like this:

CREATE INDEX index_cars_onsale_on_brand_and_model_name
  ON cars
  USING btree
  (brand COLLATE pg_catalog."default" , model_name COLLATE pg_catalog."default" )
  WHERE sales_state::text = 'onsale'::text;

Anyone seeing some big obvious issue?

EDIT:

SELECT pg_relation_size('cars'), pg_total_relation_size('cars');

pg_relation_size: 2058444800 pg_total_relation_size: 4900126720

SELECT pg_relation_size('index_cars_onsale_on_brand_and_model_name');

pg_relation_size: 46301184

SELECT avg(pg_column_size(cars)) FROM cars limit 5000;

avg: 636.9732567210792995

WITHOUT THE LIMIT:

EXPLAIN ANALYZE SELECT "cars".* FROM "cars" WHERE "cars"."sales_state" = 'onsale' AND "cars"."brand" = 'BMW' AND "cars"."model_name" = '318i' AND "cars"."has_auto_gear" = TRUE

Bitmap Heap Scan on cars  (cost=12.54..1156.95 rows=183 width=4) (actual time=17.067..55.198 rows=2096 loops=1)
  Recheck Cond: (((brand)::text = 'BMW'::text) AND ((model_name)::text = '318i'::text) AND ((sales_state)::text = 'onsale'::text))
  Filter: has_auto_gear
  ->  Bitmap Index Scan on index_cars_onsale_on_brand_and_model_name  (cost=0.00..12.54 rows=585 width=0) (actual time=15.211..15.211 rows=7411 loops=1)"
        Index Cond: (((brand)::text = 'BMW'::text) AND ((model_name)::text = '318i'::text))
Total runtime: 56.851 ms

Answer 1

While not as useful for a simple plan like this, http://explain.depesz.com is really useful. See http://explain.depesz.com/s/t4fi. Note the "stats" tab and the "options" pulldown.

Things to note about this plan:

• The estimated row count (183) is reasonably comparable to the actual row count (25). It's not hundreds of times more, nor is it 1. You're more interested in orders of magnitude when it comes to rowcount estimates, or "1 vs not 1" issues. (You don't even need "close enough for government work" accuracy - "close enough for military contracting accounting" will do). The selectivity estimate and statistics seem reasonable.

• It's using the two-column partial index provided (index scan using index_cars_onsale_on_brand_and_model_name), so it's matched the partial index condition. You can see that in the Filter: has_auto_gear. The index search condition is also shown.

• The query performance looks reasonable given that the table's row count will mean the index is fairly big, especially as it's over two columns. Matching rows will be scattered, so it's likely each row will require a separate page read too.

I see nothing wrong here. This query will likely benefit greatly from PostgreSQL 9.2's index-only scans, though.

It's possible there's some table bloat here, but given the 2-column index and the sheer number of rows the response time isn't entirely unreasonable, especially for a table with 170 (!!) columns that's likely to fit relatively few tuples into each page. If you can afford some downtime try VACUUM FULL to reorganize the table and rebuild the index. This will exclusively lock the table for some time while it rebuilds it. If you can't afford the downtime, see pg_reorg and/or CREATE INDEX CONCURRENTLY and ALTER INDEX ... RENAME TO.

You might find EXPLAIN (ANALYZE, BUFFERS, VERBOSE) more informative sometimes, as it can show buffer accesses, etc.

One option that may make this query faster (though it runs the risk of slowing other queries somewhat) is to partition the table on brand and enable constraint_exclusion. See partitioning.

Answer 2

Well... the first thing I can tell you is that your database is expecting (from the statistics) to get 183 rows. In reality it is getting 25 rows. Although that is probably not too relevant in this case (i.e. with these small amounts and no heavy operations, don't have to worry about estimating it wrongly).

A bigger problem (imho) is that a simple index lookup for 25 rows is taking 35ms. That seems a bit much. Is the database heavy enough to have at least all indexes in memory? It is not excessive though, just seems a bit slow for me.

As for looking at your explains, I would recommend using explain.depesz.com: http://explain.depesz.com/s/sA6