The gp_legacy_string_agg module is installed when you install Greenplum Database. Before you can use the function defined in the module, you must register the gp_legacy_string_agg extension in each database where you want to use the function. Refer to Installing Additional Supplied Modules for more information about registering the module.

The single-argument string_agg() function has the following signature:

string_agg( text )

You can use the function to concatenate non-null input values into a string. For example:

SELECT string_agg(a) FROM (VALUES('aaaa'),('bbbb'),('cccc'),(NULL)) g(a);
WARNING:  Deprecated call to string_agg(text), use string_agg(text, text) instead
  string_agg  
--------------
 aaaabbbbcccc
(1 row)

The function concatenates each string value until it encounters a null value, and then returns the string. The function returns a null value when no rows are selected in the query.

string_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 int, b text);
INSERT INTO table1 VALUES(4, 'aaaa'),(2, 'bbbb'),(1, 'cccc'), (3, NULL);
SELECT string_agg(b ORDER BY a) FROM table1;
WARNING:  Deprecated call to string_agg(text), use string_agg(text, text) instead
  string_agg  
--------------
 ccccbbbb
(1 row)

Greenplum Database 6 includes a native, two-argument, text input string_agg() function:

string_agg( text, text )

The following function invocation is equivalent to the single-argument string_agg() function that is provided in this module:

string_agg( text, '' )

You can use this conversion when you are ready to migrate from this contrib module.