Just maintaining a list of plpgsql functions I've created

functions.sql

-- Check for table existence
create or replace function table_exists( tbl_name text )
returns boolean as $$
begin
  return exists (
    select 1 from pg_catalog.pg_tables where tablename = tbl_name
  );
end;
$$ language plpgsql;

-- Check for column existence
create or replace function column_exists( tbl_name text, col_name text )
returns boolean as $$
begin
  return exists (
    select column_name from information_schema.columns
    where table_name = tbl_name and column_name = col_name
  );
end;
$$ language plpgsql;

-- Check constraint existence
create or replace function constraint_exists( c_type text, tbl_name text, col_name text )
returns boolean as $$
begin
  return exists (
    select 1 from
      information_schema.constraint_column_usage usage
    left join information_schema.table_constraints constraints
      on constraints.constraint_name = usage.constraint_name
    where constraints.constraint_type = c_type
      and usage.table_name = tbl_name
      and usage.column_name = col_name
  );
end;
$$ language plpgsql;

-- Add column to table
create or replace function add_column( tbl_name text, col_name text, col_type text )
returns void as $$
begin
  if exists ( select * from column_exists( tbl_name, col_name ) where column_exists = false ) then
    raise notice 'Adding column `%` to table `%`', col_name, tbl_name;
    execute 'alter table "' || tbl_name || '" add column "' || col_name || '" ' || col_type;
  end if;
end;
$$ language plpgsql;

-- Add type
create or replace function add_type( type_name text, type_def text )
returns void as $$
begin
  if not exists ( select 1 from pg_type where typname = type_name ) then
    raise notice 'Adding Type `%`', type_name;
    execute 'create type ' || type_name || ' as ' || type_def;
  end if;
end;
$$ language plpgsql;