если версия == 8.3 тогда
    \i /usr/share/postgres/8.3/contrib/hstore.sql
иначе если версия == 8.4 тогда
    \i /usr/share/postgres/8.4/contrib/hstore.sql
иначе
    create extension "hstore";
все... ;)

CREATE OR REPLACE FUNCTION system.install_extension()
  RETURNS void AS
$BODY$
DECLARE
    cur_version INT; -- Версия сервера

BEGIN

SELECT INTO cur_version setting::int FROM pg_settings WHERE name = 'server_version_num';

-- Для версии 8.3
IF cur_version > 80300 AND cur_version < 80400 THEN
    RAISE NOTICE 'INSTALL EXTENSION FOR VERSION %', cur_version;
    EXECUTE pg_read_file('/usr/share/postgresql/8.3/contrib/hstore.sql', 0, (pg_stat_file('/usr/share/postgresql/8.3/contrib/hstore.sql')).size);
-- Для версии 8.4
ELSEIF cur_version > 80400 AND cur_version < 80500 THEN
    RAISE NOTICE 'INSTALL EXTENSION FOR VERSION %', cur_version;
    EXECUTE pg_read_file('/usr/share/postgresql/8.4/contrib/hstore.sql', 0, (pg_stat_file('/usr/share/postgresql/8.4/contrib/hstore.sql')).size);
ELSEIF cur_version > 90000 THEN
    RAISE NOTICE 'INSTALL EXTENSION FOR VERSION %', cur_version;
    EXECUTE 'CREATE EXTENSION "hstore";';
ELSE
    RAISE NOTICE 'NOT SUPPORTED VERSION SERVER %', cur_version;
END IF;


END;$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;

SELECT system.install_extension();

DROP FUNCTION system.install_extension();

CREATE FUNCTION
ALTER FUNCTION
COMMENT
NOTICE:  INSTALL EXTENSION FOR VERSION 80419
 ERROR:  absolute path not allowed 
КОНТЕКСТ:  PL/pgSQL function "install_extension" line 16 at EXECUTE statement
DROP FUNCTION

CREATE TEMP TABLE x (x text);
COPY x from '/path/to/myfile.txt';
mytxt := (SELECT x from x);

CREATE FUNCTION
ALTER FUNCTION
COMMENT
NOTICE:  INSTALL EXTENSION FOR VERSION 80419
 ERROR:  extra data after last expected column 
КОНТЕКСТ:  COPY x, line 31: "   LEFTARG = hstore,"
SQL statement "COPY x from '/usr/share/postgresql/8.4/contrib/hstore.sql'"
PL/pgSQL function "install_extension" line 21 at SQL statement
DROP FUNCTION

CREATE OR REPLACE FUNCTION system.install_extension()
  RETURNS void AS
$BODY$
DECLARE
  cur_version INT; -- Версия сервера
  row_text TEXT;
  file_content TEXT;
  cursor_get_text_row CURSOR FOR
    SELECT row_string FROM text_table;

BEGIN

SELECT INTO cur_version setting::int FROM pg_settings WHERE name = 'server_version_num';

-- Для версии 8.3
IF cur_version > 80300 AND cur_version < 80400 THEN
  RAISE NOTICE 'INSTALL EXTENSION FOR VERSION %', cur_version;
  CREATE TEMP TABLE text_table (row_string text);
  COPY text_table (row_string) from '/usr/share/postgresql/8.3/contrib/hstore.sql' DELIMITER '[';

  file_content := '';
  OPEN cursor_get_text_row;
  LOOP
    FETCH cursor_get_text_row INTO row_text;
    EXIT WHEN NOT FOUND;
    file_content := file_content || row_text || E'\n';
  END LOOP;
  CLOSE cursor_get_text_row;

  EXECUTE file_content;

-- Для версии 8.4
ELSEIF cur_version > 80400 AND cur_version < 80500 THEN
  RAISE NOTICE 'INSTALL EXTENSION FOR VERSION %', cur_version;
  CREATE TEMP TABLE text_table (row_string text);
  COPY text_table (row_string) from '/usr/share/postgresql/8.4/contrib/hstore.sql' DELIMITER '[';

  file_content := '';
  OPEN cursor_get_text_row;
  LOOP
    FETCH cursor_get_text_row INTO row_text;
    EXIT WHEN NOT FOUND;
    file_content := file_content || row_text || E'\n';
  END LOOP;
  CLOSE cursor_get_text_row;

  EXECUTE file_content;

ELSEIF cur_version > 90000 THEN
    RAISE NOTICE 'INSTALL EXTENSION FOR VERSION %', cur_version;
    EXECUTE 'CREATE EXTENSION "hstore";';
ELSE
    RAISE NOTICE 'NOT SUPPORTED VERSION SERVER %', cur_version;
END IF;


END;$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;

SELECT system.install_extension();

DROP FUNCTION system.install_extension();

CREATE OR REPLACE FUNCTION system.install_extensions()
  RETURNS void AS
$BODY$
DECLARE
  cur_version INT; -- Версия сервера
  row_text_hstore TEXT;
  file_content_hstore TEXT;
  cursor_get_text_row_hstore CURSOR FOR
    SELECT row_string FROM text_table_hstore;

BEGIN

SELECT INTO cur_version setting::int FROM pg_settings WHERE name = 'server_version_num';

-- Для версии 8.x
IF cur_version > 80300 AND cur_version < 80500 THEN
  RAISE NOTICE 'INSTALL EXTENSION FOR VERSION %', cur_version;
  
  CREATE TEMP TABLE text_table_hstore (row_string text);
  
  -- Для версии 8.3
  IF cur_version > 80300 AND cur_version < 80400 THEN
    COPY text_table_hstore (row_string) from '/usr/share/postgresql/8.3/contrib/hstore.sql' DELIMITER '[';
  END IF;
  
  -- Для версии 8.4
  IF cur_version > 80400 AND cur_version < 80500 THEN
    COPY text_table_hstore (row_string) from '/usr/share/postgresql/8.4/contrib/hstore.sql' DELIMITER '[';
  END IF;

  file_content_hstore := '';
  OPEN cursor_get_text_row_hstore;
  LOOP
    FETCH cursor_get_text_row_hstore INTO row_text_hstore;
    EXIT WHEN NOT FOUND;
    file_content_hstore := file_content_hstore || row_text_hstore || E'\n';
  END LOOP;
  CLOSE cursor_get_text_row_hstore;
  
  EXECUTE file_content_hstore;

ELSEIF cur_version > 90000 THEN
    RAISE NOTICE 'INSTALL EXTENSION FOR VERSION %', cur_version;
    EXECUTE 'CREATE EXTENSION "hstore";';
ELSE
    RAISE NOTICE 'NOT SUPPORTED VERSION SERVER %', cur_version;
END IF;


END;$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;

SELECT system.install_extensions();

DROP FUNCTION system.install_extensions();