POSTGRES：函数接受哈希图或类似类型作为参数

Question

Is there a way to pass hashmap (or similar data type) to a postgres function as an argument?

I am trying to get a function that can accept different no. of arguments in different cases. And I dont want to pass nulls or 0's for those arguments that are not relevant to a particular caller.

Example of what I am trying to achieve (excuse the pseudocode)

function(hashmap map) {
condition = 'where ';
 for (entry : map) {
  condtion = condition || map.key || '=' || map.value;
 }
  sql := sql || condition;
  //execute sql
}

Is there a way to achieve this in postgres?

Answer 1

For your use case, you can use a hstore or two arrays, or 2D array. Your example is nice demonstration of SQL injection, so you should not to forget on necessary escaping.

CREATE OR REPLACE FUNCTION hstore_params(filters hstore)
RETURNS text AS $$
BEGIN
  RETURN 'SELECT * FROM some_table ' ||
            coalesce ('WHERE ' ||
            ( SELECT string_agg(quote_ident(key) || ' = ' || quote_literal(value), ' and ') 
                 FROM each('c1 => Ahoj, c2 => Nazdar'::hstore). '') );
END;
$$ LANGUAGE plpgsql;

postgres=# SELECT hstore_params('c1 => Ahoj, c2 => Nazdar');
                        hstore_params                         
--------------------------------------------------------------
 SELECT * FROM some_table WHERE c1 = 'Ahoj' and c2 = 'Nazdar'
(1 row)

Next possibility is usage of function default parameters. It is my personal favorite:

CREATE OR REPLACE FUNCTION hstore_params(c1 text DEFAULT NULL, c2 text DEFAULT NULL)
RETURNS text AS $$
BEGIN 
  EXECUTE 'SELECT * 
             FROM xx
            WHERE (c1 = $1 OR c1 IS NULL) 
              AND (c2 = $2 OR c2 IS NULL)'
    USING c1, c2;
  RETURN 'ok';
END;
$$ LANGUAGE plpgsql;

postgres=# SELECT hstore_params();
 hstore_params 
---------------
 ok
(1 row)

postgres=# SELECT hstore_params('Ahoj','Nazdar');
 hstore_params 
---------------
 ok
(1 row)

postgres=# SELECT hstore_params('Ahoj');
 hstore_params 
---------------
 ok
(1 row)

postgres=# SELECT hstore_params(c2 := 'Ahoj');
 hstore_params 
---------------
 ok
(1 row)