Postgresql – How to Choose Between VALUES and SELECT for INSERT

SELECT INTO is usually used to select specific set of data into a table, esp., during scenarios when the data in the table is the priority and not the constraints. It automatically creates a table if there is no such table already. But, INSERT INTO is used when you already have a table that has specific defined constraints and need to add data from a different table.

You have an unresolved naming conflict.

You must be using an old version of Postgres without declaring it. Or you are operating with non-default configuration setting.

Here you declare a variable named measurement_id:

DECLARE
    measurement_id              INTEGER;

It's a folly to use ambiguous variable names to begin with. If you do it anyway, you must know what you are doing. I make it a habbit to prepend variable names with an underscore unlike column names, like _measurement_id.

The later SELECT statement is ambiguous:

ORDER BY measurement_id

This would raise an error message in modern PostgreSQL with default configuration. Per the documentation:

By default, PL/pgSQL will report an error if a name in a SQL statement could refer to either a variable or a table column.

And:

To change this behavior on a system-wide basis, set the configuration parameter plpgsql.variable_conflict to one of error, use_variable, or use_column (where error is the factory default). This parameter affects subsequent compilations of statements in PL/pgSQL functions, but not statements already compiled in the current session. Because changing this setting can cause unexpected changes in the behavior of PL/pgSQL functions, it can only be changed by a superuser.

In Postgres older than 9.0 this would be resolved to mean the variable. Per the documentation

In such cases you can specify that PL/pgSQL should resolve ambiguous references as the variable (which is compatible with PL/pgSQL's behavior before PostgreSQL 9.0)

Bold emphasis mine.
This would result in arbitrary results, since the sort order is now undetermined.

Audited Function

CREATE OR REPLACE FUNCTION insaft_function()
   RETURNS TRIGGER AS
$func$
DECLARE
   _measurement_id          integer;
   _file_header_index_start integer := TG_ARGV[0]::int;
   _file_header_index_end   integer := TG_ARGV[1]::int; 
BEGIN     

   SELECT a.measurement_id   INTO _measurement_id
   FROM   measurements a
   ORDER  BY a.measurement_id DESC  -- you had ambiguity here!
   LIMIT  1;

   IF TG_OP = 'INSERT' THEN  -- noise if only used in AFTER INSERT trigger
      INSERT INTO file_headers (measurement_id, file_header_index_start
                                              , file_header_index_end)
      VALUES (_measurement_id, _file_header_index_start, _file_header_index_end); 
   END IF;

   RETURN NULL; -- result is ignored since this is an AFTER trigger
END
$func$ LANGUAGE plpgsql;

Note how I named it insaft_function(), since this is only to be used in an AFTER INSERT trigger.

Trigger:

CREATE TRIGGER insaft_measurement_ids
AFTER INSERT ON measurements
FOR EACH ROW EXECUTE PROCEDURE insaft_function(1, 666);

But for the provided setup, you can radically simplify the function:

CREATE OR REPLACE FUNCTION insaft_function()
   RETURNS TRIGGER AS
$func$
BEGIN     
   INSERT INTO file_headers (measurement_id, file_header_index_start
                                           , file_header_index_end)
   VALUES (NEW.measurement_id, TG_ARGV[0]::int, TG_ARGV[1]::int);

   RETURN NULL;  -- result ignored since this is an AFTER trigger
END
$func$ LANGUAGE plpgsql;