Question

I have a column of Int Array. I want to add another column to the table, that always shows the number elements in that array for that row. It should update this value automatically. Is there a way to embedd a function as default value? If so, how would this function know where to pick its argument (the int array column/row number).

Answer 1

In a normalized table you would not include this functionally dependent and redundant information as a separate column.

It is easy and fast enough to compute it on the fly:

SELECT array_dims ('{1,2,3}'::int[]);

Or:

SELECT array_length('{1,2,3}'::int[], 1);

array_length() has been introduced with PostgreSQL 8.4. Maybe an incentive to upgrade? 8.3 is going out of service soon.

With Postgres 8.3 you can use:

SELECT array_upper('{1,2,3}'::int[], 1);

But that’s inferior, because the array index can start with any number, if entered explicitly. array_upper() would not tell the actual length then, you would have to subtract array_lower() first. Also note, that in PostgreSQL arrays can always contain multiple dimensions, regardless of how many dimensions have been declared. I quote the manual here:

The current implementation does not enforce the declared number of
dimensions either. Arrays of a particular element type are all
considered to be of the same type, regardless of size or number of
dimensions. So, declaring the array size or number of dimensions in
CREATE TABLE is simply documentation; it does not affect run-time
behavior.

(True for 8.3 and 9.1 alike.) That’s why I mentioned array_dims() first, to give a complete picture.

Details about array functions in the manual.

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You may want to create a view to include that functionally dependent column:

CREATE VIEW v_tbl AS
SELECT arr_col, array_length(arr_col, 1) AS arr_len
FROM   tbl;