CHECK constraint for array column to verify length > 0

I'm playing with postgres table validation rules and trying to set a CHECK constraint for an array column. An idea is to allow only arrays with length > 0.

Here is how I want to implement it:

create table words_table (
  id serial primary key, 
  words varchar(20)[] CHECK (array_length(words, 1) > 0)
);

But looks like it doesn't work. O_o

insert into words_table (words) values ('{}');
//INSERT 0 1

How to implement such a constraint?

The crucial point you may not be aware of, quoting the manual:

It should be noted that a check constraint is satisfied if the check
expression evaluates to true or the null value.

Bold emphasis mine.

Just rule out empty arrays:

CHECK (words <> '{}')

This still allows words to be null. You may want to add a NOT NULL constraint.

Or catch nullin the CHECK constraint, too

CHECK ((words <> '{}') IS TRUE)

StackExchange Database Administrators Q#183891, answer score: 9