Elegant way of handling PostgreSQL exceptions?

Question

In PostgreSQL, I would like to create a safe-wrapping mechanism which returns empty result if an exception occurs. Consider the following:

SELECT * FROM myschema.mytable;

I could do the safe-wrapping in the client application:

try {
    result = execute_query('SELECT value FROM myschema.mytable').fetchall();
}
catch(pg_exception) {
    result = []
}

But could I do such a thing in SQL directly? I would like to make the following code work, but it seems like it should by put into DO $$ ... $$ block and here I'm getting lost.

BEGIN
    SELECT * FROM myschema.mytable;
EXCEPTION WHEN others THEN
    SELECT unnest(ARRAY[]::TEXT[])
END

Answer 1

Exception handling in PL/pgSQL

PL/pgSQL code is always wrapped into a BEGIN ... END block. That can be inside the body of a DO statement or a function. Blocks can be nested inside - but they cannot exist outside, don't confuse it with plain SQL.

Each block can optionally contain an EXCEPTION clause for handling exceptions, but functions that need to trap exceptions are more expensive, so it's best to avoid exceptions a priori. Postgres needs to prepare for the possibility of rolling back to a point in the transaction before the exception happened, similar to an SQL SAVEPOINT. The manual:

A block containing an EXCEPTION clause is significantly more expensive to enter and exit than a block without one. Therefore, don't use EXCEPTION without need.

Example:

• Is SELECT or INSERT in a function prone to race conditions?

How to avoid an exception in the example

A DO statement can't return anything. Create a function that takes table and schema name as parameters and returns whatever you want:

CREATE OR REPLACE FUNCTION f_tbl_value(_tbl text, _schema text = 'public')
  RETURNS TABLE (value text)
  LANGUAGE plpgsql AS
$func$
DECLARE
   _t regclass := to_regclass(_schema || '.' || _tbl);
BEGIN
   IF _t IS NULL THEN
      value := ''; RETURN NEXT;    -- return single empty string
   ELSE
      RETURN QUERY EXECUTE
      'SELECT value FROM ' || _t;  -- return set of values
   END IF;
END
$func$;

Call:

SELECT * FROM f_tbl_value('my_table');

Or:

SELECT * FROM f_tbl_value('my_table', 'my_schema');

Assuming you want a set of rows with a single text column or an empty string if the table does not exist.

Also assuming that a column value exists if the given table exists. You could test for that, too, but you didn't ask for that.

Both input parameters are only case sensitive if double-quoted. Just like identifiers are handled in SQL statements.

The schema name defaults to 'public' in my example. Adapt to your needs. You could even ignore the schema completely and default to the current search_path.

to_regclass() is new in Postgres 9.4. For older versions substitute:

IF EXISTS (
   SELECT FROM information_schema.tables 
   WHERE  table_schema = _schema
   AND    table_name = _tbl
   ) THEN ...

This is actually more accurate, because it tests exactly what you need. More options and detailed explanation:

• Table name as a PostgreSQL function parameter

Always defend against SQL injection when working with dynamic SQL! The cast to regclass does the trick here. More details:

• How to check if a table exists in a given schema

Answer 2

If you are selecting only one column then the COALESCE() function should be able to do the trick for you

SELECT COALESCE( value, '{}'::text[] ) FROM myschema.mytable

If you require more rows you may require to create a function with types.