PostgreSQL return exact or closest date to queried date

Question

I have the following postgresql syntax that returns values WHERE session_date matches $date_string

Problem is that sometimes the $date_string will not be available in th

Answer 1

If you want the closest date before, do it this way:

SELECT year, session_date
FROM calendar_dates
WHERE session_date < '$date_string'
ORDER BY session_date DESC
LIMIT 1;

The closest date after uses similar logic.

For the closest on either side:

SELECT year, session_date
FROM calendar_dates
ORDER BY abs(session_date - date '$date_string') 
LIMIT 1;

Answer 2

Using btree_gist and knn

Using this method you can find the nearest event with an index.

CREATE EXTENSION btree_gist;
CREATE TABLE foo ( id serial, ts timestamp );

INSERT INTO foo (ts)
VALUES
  ('2017-06-02 03:09'),
  ('2016-06-02 03:09'),
  ('1900-06-02 03:09'),
  ('1954-06-02 03:09');

CREATE INDEX ON foo USING gist(ts);

SELECT *
FROM foo
ORDER BY '1950-06-02 03:09' <-> ts
LIMIT 1;

Pg 11

Coming some time in the distant future... with knn/btree