Inconsistent query times and spiky CPU with GIN tsvector search

Hello all,

We are running postgresql 9.4 and we have a table where we do some full-text searching using a GIN index on a tsvector column:

CREATE TABLE public.location_search
(
id bigint NOT NULL DEFAULT nextval('location_search_id_seq'::regclass),
<snip some columns>…
search_field_tsvector tsvector
)

and

CREATE INDEX location_search_tsvector_idx
ON public.location_search USING gin
(search_field_tsvector)
TABLESPACE pg_default;

The search_field_tsvector column contains the data from the location's name and address:

to_tsvector('pg_catalog.english', COALESCE(NEW.name, '')) || to_tsvector(COALESCE(address, ''))

This setup has been running very well, but as our load is getting heavier, the performance seems to be getting much more inconsistent. Our searches are run on a dedicated read replica, so this server is only doing queries against this one table. IO is very low, indicating to me that the data is all in memory. However, we're getting some queries taking upwards of 15-20 seconds, while the average is closer to 1 second.

A sample query that's running slowly is

explain (analyze, buffers)
SELECT ls.location AS locationId FROM location_search ls
WHERE ls.client = 1363
AND ls.favorite = TRUE
AND search_field_tsvector @@ to_tsquery('CA-94:* &E &San:*')
LIMIT 4;

And the explain analyze is:

Limit (cost=39865.85..39877.29 rows=1 width=8) (actual time=4471.120..4471.120 rows=0 loops=1)
Buffers: shared hit=25613
-> Bitmap Heap Scan on location_search ls (cost=39865.85..39877.29 rows=1 width=8) (actual time=4471.117..4471.117 rows=0 loops=1)
Recheck Cond: (search_field_tsvector @@ to_tsquery('CA-94:* &E &San:*'::text))
Filter: (favorite AND (client = 1363))
Rows Removed by Filter: 74
Heap Blocks: exact=84
Buffers: shared hit=25613
-> Bitmap Index Scan on location_search_tsvector_idx (cost=0.00..39865.85 rows=6 width=0) (actual time=4470.895..4470.895 rows=84 loops=1)
Index Cond: (search_field_tsvector @@ to_tsquery('CA-94:* &E &San:*'::text))
Buffers: shared hit=25529
Planning time: 0.335 ms
Execution time: 4487.224 ms

I'm a little bit at a loss to where to start at this - any suggestions would be hugely appreciated!

Thanks,
Scott

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