Using Ecto for Postgres fulltext search on GIN indexes - postgresql

Question

I have a simple model:
schema "torrents" do
field :name, :string
field :magnet, :string
field :leechers, :integer
field :seeders, :integer
field :source, :string
field :filesize, :string
timestamps()
end
And I want to search based on the name. I added the relevant extensions and indexes to my database and table.
def change do
create table(:torrents) do
add :name, :string
add :magnet, :text
add :leechers, :integer
add :seeders, :integer
add :source, :string
add :filesize, :string
timestamps()
end
execute "CREATE EXTENSION pg_trgm;"
execute "CREATE INDEX torrents_name_trgm_index ON torrents USING gin (name gin_trgm_ops);"
create index(:torrents, [:magnet], unique: true)
end
I'm trying to search using the search term, but I always get zero results.
def search(query, search_term) do
from(u in query,
where: fragment("? % ?", u.name, ^search_term),
order_by: fragment("similarity(?, ?) DESC", u.name, ^search_term))
end
SELECT t0."id", t0."name", t0."magnet", t0."leechers", t0."seeders", t0."source",
t0."filesize", t0."inserted_at", t0."updated_at" FROM "torrents"
AS t0 WHERE (t0."name" % $1) ORDER BY similarity(t0."name", $2) DESC ["a", "a"]
Is something wrong with my search function?

Answer 1

My initial guess is that because you're using the % operator, the minimum limit to match is too high for your queries. This limit defaults to 0.3 (meaning that the strings' trigrams are 30% similar). If this threshold isn't met, no results will be returned.
If that is the issue, this threshold is configurable in a couple of ways. You can either use set_limit (docs here), or set the limit on a per query basis.
The set_limit option can be a bit of a hassle, as it needs to be set per connection every time. Ecto (through db_connection) has an option to set a callback function for after_connect (docs here).
To change the limit per query, you can use the similarity function in the where clause, like this:
def search(query, search_term, limit = 0.3) do
from(u in query,
where: fragment("similarity(?, ?) > ?", u.name, ^search_term, ^limit),
order_by: fragment("similarity(?, ?) DESC", u.name, ^search_term))
end
To start, I would try that with a limit of zero to see if you get any results.