Friday, September 11, 2015

Passing Arrays to a PostgreSQL PL/pgSQL Function

It can be handy to pass a collection of strings to a PL/pgSQL stored function via a PostgreSQL array. This is generally a very easy thing to accomplish, but this post demonstrates a couple of nuances to be aware of when passing an array to a PL/pgSQL function from JDBC or psql.

The next code listing is for a contrived PL/pgSQL stored function that will be used in this post. This function accepts an array of text variables, loops over them based on array length, and reports these strings via the PL/pgSQL RAISE statement.

CREATE OR REPLACE FUNCTION printStrings(strings text[]) RETURNS void AS $printStrings$
   number_strings integer := array_length(strings, 1);
   string_index integer := 1;
   WHILE string_index <= number_strings LOOP
      RAISE NOTICE '%', strings[string_index];
      string_index = string_index + 1;
$printStrings$ LANGUAGE plpgsql;

The above PL/pgSQL code in file printStrings.sql can executed in psql with \ir as shown in the next screen snapshot.

The syntax for invoking a PL/pgSQL stored function with an array as an argument is described in the section "Array Value Input" in the PostgreSQL Arrays documentation. This documentation explains that "general format of an array constant" is '{ val1 delim val2 delim ... }' where delim is a delimited of comma (,) in most cases. The same documentation shows an example: '{{1,2,3},{4,5,6},{7,8,9}}'. This example provides three arrays of integral numbers with three integral numbers in each array.

The array literal syntax just shown is straightforward to use with numeric types such as the integers in the example shown. However, for strings, there is a need to escape the quotes around the strings because there are already quotes around the entire array ('{}'). This escaping is accomplished by surrounding each string in the array with two single quotes on each side. For example, to invoke the stored function just shown on the three strings "Inspired", "Actual", and "Events", the following syntax can be used in psql: SELECT printstrings('{''Inspired'', ''Actual'', ''Events''}'); as shown in the next screen snapshot.

Arrays can be passed to PL/pgSQL functions from Java code as well. This provides an easy approach for passing Java collections to PL/pgSQL functions. The following Java code snippet demonstrates how to call the stored function shown earlier with JDBC. Because this stored function returns void (it's more like a stored procedure), the JDBC code does not need to invoke any CallableStatement's overridden registerOutParameter() methods.

JDBC Code Invoking Stored Function with Java Array
final CallableStatement callable =
   connection.prepareCall("{ call printstrings ( ? ) }");
final String[] strings = {"Inspired", "Actual", "Events"};
final Array stringsArray = connection.createArrayOf("varchar", strings);
callable.setArray(1, stringsArray);

Java applications often work more with Java collections than with arrays, but fortunately Collection provides the toArray(T[]) for easily getting an array representation of a collection. For example, the next code listing is adapted from the previous code listing, but works against an ArrayList rather than an array.

JDBC Code Invoking Stored Function with Java Collection
final CallableStatement callable =
   connection.prepareCall("{ call printstrings ( ? ) }");
final ArrayList<String> strings = new ArrayList<>();
final Array stringsArray =
      strings.toArray(new String[strings.size()]));
callable.setArray(1, stringsArray);


The ability to pass an array as a parameter to a PostgreSQL PL/pgSQL stored function is a straightforward process. This post specifically demonstrated passing an array of strings (including proper escaping) to a PL/pgSQL stored function from psql and passing an array of Strings to a PL/pgSQL stored function from JDBC using java.sql.Array and Connection.createArrayOf(String, Object[]).

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