BUG #14729: Between operator is slow when same value used for low and high margin

The following bug has been logged on the website:

Bug reference: 14729
Logged by: Pavel Tavoda
Email address: pavel(dot)tavoda(at)gmail(dot)com
PostgreSQL version: 9.4.10
Operating system: Linux
Description:

When using JOIN with BETWEEN with same value for low and high marging query
is taking more than 40 times longer. Amount of result rows is aroung
20.000.

SELECT count(*) FROM contractportfolio cp JOIN contract co ON cp.contract =
co.id WHERE validfor between '2017-05-30' AND '2017-05-31';
Time: 0.473

SELECT count(*) FROM contractportfolio cp JOIN contract co ON cp.contract =
co.id WHERE validfor between '2017-05-31' AND '2017-05-31';
Time: 19.172

SELECT count(*) FROM contractportfolio cp JOIN contract co ON cp.contract =
co.id WHERE validfor = '2017-05-31';
Time: 0.467