How can this simple query take so long? - postgresql

Question

=> SELECT * FROM "tags" WHERE ("kind" = 'View') ORDER BY "name";
Time: 278.318 ms
The tags table contains 358 rows. All of them are views at the moment.
Column | Type | Modifiers
-------------+--------------------------+-------------------------------------
id | uuid | not null default uuid_generate_v4()
name | text | not null
slug | text | not null
kind | text | not null
external_id | text |
created_at | timestamp with time zone | not null default now()
updated_at | timestamp with time zone |
filter | json |
Indexes:
"tags_pkey" PRIMARY KEY, btree (id)
"tags_kind_index" btree (kind)
"tags_name_index" btree (name)
Analyze says:
QUERY PLAN
----------------------------------------------------------------------------------------------------------
Sort (cost=9.29..9.47 rows=358 width=124) (actual time=0.654..0.696 rows=358 loops=1)
Sort Key: name
Sort Method: quicksort Memory: 75kB
-> Seq Scan on tags (cost=0.00..6.25 rows=358 width=124) (actual time=0.006..0.108 rows=358 loops=1)
Filter: (kind = 'View'::text)
Total runtime: 0.756 ms
(6 rows)

Answer 1

Did you run analyze tags? It will update the table's statistics.
First if all the kind values are 'view' then an index on that column is useless. The index will only be used if the column's cardinality is high otherwise it is cheaper to do a sequential scan on the table.
Second with only 358 rows it is cheaper to do a sequential scan anyway.