Why isn't my PostgreSQL array index getting used (Rails 4)?
I've got a PostgreSQL array of strings as a column in a table. I created an index using the GIN method. But ANY queries won't use the index (instead, they're doing a sequential scan of the whole table with a filter). What am I missing?
Here's my migration:
class CreateDocuments < ActiveRecord::Migration
def up
create_table :documents do |t|
t.string :title
t.string :tags, array: true, default: []
t.timestamps
end
add_index :documents, :tags, using: 'gin'
(1..100000).each do |i|
tags = []
tags << 'even' if (i % 2) == 0
tags << 'odd' if (i % 2) == 1
tags << 'divisible by 3' if (i % 3) == 0
tags << 'divisible by 4' if (i % 4) == 0
tags << 'divisible by 5' if (i % 5) == 0
Document.create(
title: i,
tags: tags
)
end
end
def down
drop_table :documents
end
end
Here's my query, with the resulting number of rows.
Document.where("'divisible by 5' = ANY (tags)").explain
Document Load (249.8ms) SELECT "documents".* FROM "documents" WHERE ('divisible by 5' = ANY (tags))
D, [2014-03-07T17:09:49.689709 #41937] DEBUG -- : Document Load (249.8ms) SELECT "documents".* FROM "documents" WHERE ('divisible by 5' = ANY (tags))
=> EXPLAIN for: SELECT "documents".* FROM "documents" WHERE ('divisible by 5' = ANY (tags))
QUERY PLAN
-----------------------------------------------------------------
Seq Scan on documents (cost=0.00..3500.00 rows=20057 width=69)
Filter: ('divisible by 5'::text = ANY ((tags)::text[]))
(2 rows)
Document.where("'divisible by 5' = ANY (tags)").length
Document Load (258.0ms) SELECT "documents".* FROM "documents" WHERE ('divisible by 5' = ANY (tags))
D, [2014-03-07T17:09:55.536517 #41937] DEBUG -- : Document Load (258.0ms) SELECT "documents".* FROM "documents" WHERE ('divisible by 5' = ANY (tags))
=> 20000
To work with a GIN index use the <@ ("is contained by") operator instead of the ANY construct.
The manual states here that default GIN indexes currently only support these operators (additional functionality is shipped with extensions):
<@
@>
=
&&
So try this query:
Document.where("'{divisible by 5}' <@ tags").explain
Note that the left hand side needs to be in array notation , too, even if it's a single element. The operator <@ works for arrays. Hence '{divisible by 5}' .