How to Retrieve Multiple Fields as a Single Record in PostgreSQL Using PL/pgSQL?

Retrieving Multiple Fields as a Record in PostgreSQL Using PL/pgSQL

When working with database interactions, it often becomes necessary to fetch data from multiple tables and present it as a unified record. PostgreSQL allows this functionality using the RECORD type in PL/pgSQL, enabling developers to retrieve fields from different tables as fields within a single record.

Returning Fields from Different Tables as a Single Record

To retrieve fields from multiple tables as fields in a single record, PL/pgSQL uses the RECORD type. Here's a modified version of your example:

CREATE OR REPLACE FUNCTION get_object_fields(name text)
  RETURNS RECORD AS $$
BEGIN
  -- Fetch fields f1, f2, and f3 from table t1
  -- Fetch fields f4, f5 from table t2
  -- Fetch fields f6, f7, and f8 from table t3

  -- Create a record to store the fetched fields
  RETURN RECORD(
    f1 TEXT,
    f2 TEXT,
    f3 TEXT,
    f4 TEXT,
    f5 TEXT,
    f6 TEXT,
    f7 TEXT,
    f8 TEXT
  );
END
$$ language plpgsql;

Handling Rows with Multiple Fields from a Single Table

When dealing with situations where certain fields may be stored across multiple rows in a single table, the RECORD type can still be used effectively. In your example with the 'user' table, you can modify the code as follows:

CREATE OR REPLACE FUNCTION get_two_users_from_school(schoolid int)
  RETURNS my_type AS $$
DECLARE
  result my_type;
  temp_result RECORD;
  user1 RECORD;
  user2 RECORD;
BEGIN
  SELECT id, name INTO temp_result FROM user WHERE school_id = schoolid LIMIT 2;

  -- Split the fetched rows into individual user records
  user1.user1_id := temp_result.id;
  user1.user1_name := temp_result.name;

  -- Handle the second row if available
  IF FOUND THEN
    user2.user2_id := temp_result.id;
    user2.user2_name := temp_result.name;
  END IF;

  -- Return the combined user record
  result.user1_id := user1.user1_id;
  result.user1_name := user1.user1_name;
  result.user2_id := user2.user2_id;
  result.user2_name := user2.user2_name;

  RETURN result;
END
$$ language plpgsql;

Polymorphic Result Handling

While using CREATE TYPE may seem reasonable for returning polymorphic results, PL/pgSQL provides a more flexible and robust approach. By abusing the RECORD type, you can return a varying number of columns based on the input, making it suitable for handling optional error messages or additional information.

Here's an example:

CREATE FUNCTION test_ret(a TEXT, b TEXT) RETURNS RECORD AS $$
DECLARE
  ret RECORD;
BEGIN
  IF LENGTH(a) < LENGTH(b) THEN
    ret := (TRUE, a || b, 'a shorter than b');
  ELSE
    ret := (FALSE, b || a);
  END IF;
  RETURN ret;
END;
$$ LANGUAGE plpgsql;

This function dynamically returns either two or three columns depending on the input and can be used for various scenarios, such as returning success status and optional error messages.

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