The gp_array_agg module is installed when you install Greenplum Database. Before you can use the aggregate function defined in the module, you must register the gp_array_agg extension in each database where you want to use the function:

CREATE EXTENSION gp_array_agg;

Refer to Installing Additional Supplied Modules for more information.

The gp_array_agg() function has the following signature:

gp_array_agg( anyelement )

You can use the function to create an array from input values, including nulls. For example:

SELECT gp_array_agg(a) FROM t1;
   gp_array_agg   
------------------
 {2,1,3,NULL,1,2}
(1 row)

gp_array_agg() assigns each input value to an array element, and then returns the array. The function returns null rather than an empty array when there are no input rows.

gp_array_agg() produces results that depend on the ordering of the input rows. The ordering is unspecified by default; you can control the ordering by specifying an ORDER BY clause within the aggregate. For example:

CREATE TABLE table1(a int4, b int4);
INSERT INTO table1 VALUES (4,5), (2,1), (1,3), (3,null), (3,7);
SELECT gp_array_agg(a ORDER BY b NULLS FIRST) FROM table1;
  gp_array_agg  
--------------
 {3,2,1,4,7}
(1 row)

Refer to Aggregate Functions in the PostgreSQL documentation for more information about aggregates.