PostgreSQL gist index - postgresql

Question

I have a table with two date like dateTo and dateFrom, i would like use daterange approach in queries and a gist index, but it seem doesn't work. The table looks like:
CREATE TABLE test (
id bigeserial,
begin_date date,
end_date date
);
CREATE INDEX "idx1"
ON test
USING gist (daterange(begin_date, end_date));
Then when i try to explain a query like:
SELECT t.*
FROM test t
WHERE daterange(t.begin_date,t.end_date,'[]') && daterange('2015-12-30 00:00:00.0','2016-10-28 00:00:00.0','[]')
i get a Seq Scan.
Is this usage of gist index wrong, or is this scenario not feasible?

Answer 1

You have an index on the expression daterange(begin_date, end_date), but you query your table with daterange(begin_date, end_date, '[]') && .... PostgreSQL won't do math instead of you. To re-phrase your problem, it is like you're indexing (int_col + 2) and querying WHERE int_col + 1 > 2. Because the two expressions are different, the index will not be used in any circumstances. But as you can see, you can do the math (i.e. re-phrase the formula) sometimes.
You'll either need:
CREATE INDEX idx1 ON test USING gist (daterange(begin_date, end_date, '[]'));
Or:
CREATE INDEX idx2 ON test USING gist (daterange(begin_date, end_date + 1));
Note: both of them creates a range which includes end_date. The latter one uses the fact that daterange is discrete.
And use the following predicates for each of the indexes above:
WHERE daterange(begin_date, end_date, '[]') && daterange(?, ?, ?)
Or:
WHERE daterange(begin_date, end_date + 1) && daterange(?, ?, ?)
Note: the third parameter of the range constructor on the right side of && does not matter (in the context of index usage).