What are the proper use-cases for the PostgreSQL Array Datatype?

It seems to me that the functionality of the PostgreSQL array datatype overlaps a lot with the standard one-to-many and many-to-many relationships.

For example, a table called users could have an array field called "favorite_colors", or there could be a separate table called开发者_JS百科 "favorite_colors" and a join table between "users" and "favorite_colors".

In what cases is the array datatype OK to use instead of a full-blown join?

An array should not be used similar to a relation. It should rather contain indexed values that relate to one row very tightly. For example if you had a table with the results of a football match, than you would not need to do

id team1 team2 goals1 goals2

but would do

id team[2] goals[2]

Because in this example, most would also consider normalizing this into two tables would be silly.

So all in all I would use it in cases where you are not interested in making relations and where you else would add fields like field1 field2 field3.

One incredibly handy use case is tagging:

CREATE TABLE posts (
    title TEXT,
    tags TEXT[]
);

-- Select all posts with tag 'kitty'
SELECT * FROM posts WHERE tags @> '{kitty}';

I totally agree with @marc. Arrays are to be used when you are absolutely sure you don't need to create any relationship between the items in the array with any other table. It should be used for a tightly coupled one to many relationship.
A typical example is creating a multichoice questions system. Since other questions don't need to be aware of the options of a question, the options can be stored in an array.
e.g

CREATE TABLE Question (
  id integer PRIMARY KEY,
  question TEXT,
  options VARCHAR(255)[],
  answer VARCHAR(255)
)  

This is much better than creating a question_options table and getting the options with a join.

The Postgresql documentation gives good examples:

  CREATE TABLE sal_emp (
     name            text,
     pay_by_quarter  integer[],
     schedule        text[][]
 );

The above command will create a table named sal_emp with a column of type text (name), a one-dimensional array of type integer (pay_by_quarter), which represents the employee's salary by quarter, and a two-dimensional array of text (schedule), which represents the employee's weekly schedule.

Or, if you prefer:

 CREATE TABLE tictactoe (
     squares   integer[3][3] );

If I want to store some similar type of set of data, and those data don't have any other attribute.

I prefer to use arrays.

One example is :

Storing contact numbers for a user

So, when we want to store contact number, usually main one and a alternate one, in such case

I prefer to use array.

CREATE TABLE students (  
    name text,
    contacts varchar ARRAY -- or varchar[]
);

But if these data have additional attributes, say storing cards. A card can have expiry date and other details.

Also, storing tags as an array a bad idea. A tag can be associated to multiple posts.

Don't use arrays in such cases.