СREATE TABLE period_avg_params
(
  datetime_id int8 NOT NULL,
  param_id int2 NOT NULL,
  event_datetime timestamp NOT NULL,
  value float4 NOT NULL,
  CONSTRAINT period_avg_params_pkey PRIMARY KEY (datetime_id, param_id, event_datetime),
  CONSTRAINT period_avg_params_dt_fkey FOREIGN KEY (datetime_id) REFERENCES period_avg_params_times (datetime_id) ON UPDATE RESTRICT ON DELETE CASCADE,
) 
WITH OIDS;

CREATE INDEX period_avg_params_dt_fkey_index
  ON period_avg_params
  USING btree
  (datetime_id);

CREATE INDEX period_avg_params_param_fkey_index
  ON period_avg_params
  USING btree
  (param_id);

event_datetime |param_id1|param_id2|...|param_id500|
---------------|---------|---------|---|-----------|
время1         |  10       |  0 . 2      |...|  7 . 2        |
время2         |  23       |  0 . 3      |...|  3 . 2        |
....................................................
времяN         |  23       |  0 . 3      |...|  3 . 2        |

--такая функуция:
CREATE OR REPLACE FUNCTION coalesce_ag_f(anyelement, anyelement)
  RETURNS anyelement AS
$BODY$
	select coalesce($ 1 ,$ 2 );
$BODY$
  LANGUAGE 'sql' VOLATILE;

--агрегат:
CREATE AGGREGATE coalesce_ag(
  BASETYPE=anyelement,
  SFUNC=coalesce_ag_f,
  STYPE=anyelement,
  FFUNC="-"
);
ALTER AGGREGATE coalesce_ag OWNER TO postgres;

--и собственно сам запрос:
SELECT 
  event_datetime, 
  coalesce_ag("1") as "f",
  coalesce_ag("2") as "U_A",
  coalesce_ag("3") as "U_B",
  coalesce_ag("4") as "U_C",
FROM 
(SELECT DISTINCT event_datetime,
    case param_id when '1' then value else NULL end as "1",
    case param_id when '2' then value else NULL end as "2",
    case param_id when '3' then value else NULL end as "3",
    case param_id when '4' then value else NULL end as "4"
FROM period_avg_params
WHERE datetime_id =  32 ) AS t
GROUP BY event_datetime ORDER BY event_datetime;

"GroupAggregate  (cost=1037.03..1272.06 rows=200 width=476) (actual time=9183.000..53607.000 rows=87 loops=1)"
"  ->  Subquery Scan t  (cost=1037.03..1127.12 rows=293 width=476) (actual time=8663.000..10497.000 rows=10266 loops=1)"
"        ->  Unique  (cost=1037.03..1124.19 rows=293 width=14) (actual time=8603.000..9355.000 rows=10266 loops=1)"
"              ->  Sort  (cost=1037.03..1037.76 rows=293 width=14) (actual time=8603.000..8784.000 rows=46980 loops=1)"
"                    Sort Key: event_datetime, CASE param_id WHEN 2302::smallint THEN value ELSE NULL::real END, CASE param_id WHEN 2303::smallint THEN value ELSE NULL::real END, CASE param_id WHEN 2304::smallint THEN value ELSE NULL::real END, CASE param_i (..)"
"                    ->  Index Scan using period_avg_params_dt_fkey_index on period_avg_params  (cost=0.00..1025.02 rows=293 width=14) (actual time=0.000..5047.000 rows=46980 loops=1)"
"                          Index Cond: (datetime_id = 32)"
"Total runtime: 53627.000 ms"

 ArrayList rows=new ArrayList();
 ArrayList cols=new ArrayList();
 // схема a_table ( data int, param varchar, value varchar, unique(data, param) )
 // в rows загоняем результат запроса "select distinct data from a_table order by 1"
 // в cols загоняем результат запроса "select distinct param from a_table order by 1"
 Object[][] data=new Object[rows.Count][cols.Count]; // табличка с данными
 NpgsqlDataAdapter  myCommand = new NpgsqlDataAdapter("select data, param, value from a_table", myConnection);
 DataSet ds=new DataSet();
 myCommand.Fill(ds,"a_table");
 // заполнение таблички данными
 for(DataRow row in ds.Tables["a_table"].Rows)
 {
        int i=rows.IndexOf(row["data"]);
        int j=cols.IndexOf(row["param"]);
        data[i][j]=row["value"];
 }

--и собственно сам запрос:
SELECT 
  event_datetime, 
  coalesce_ag(case param_id when '1' then value else NULL end) as "f",
  coalesce_ag(case param_id when '2' then value else NULL end) as "U_A",
  coalesce_ag(case param_id when '3' then value else NULL end) as "U_B",
  coalesce_ag(case param_id when '4' then value else NULL end) as "U_C",
FROM 
  period_avg_params
WHERE datetime_id =  32 
GROUP BY event_datetime ORDER BY event_datetime;

-- результат EXPLAIN ANALYZE SELECT... для 4 записей
"Sort  (cost=943.03..943.04 rows=2 width=14) (actual time=6510.000..6510.000 rows=87 loops=1)"
"  Sort Key: event_datetime"
"  ->  HashAggregate  (cost=942.98..943.02 rows=2 width=14) (actual time=6510.000..6510.000 rows=87 loops=1)"
"        ->  Index Scan using period_avg_params_dt_fkey_index on period_avg_params  (cost=0.00..939.32 rows=293 width=14) (actual time=0.000..590.000 rows=46980 loops=1)"
"              Index Cond: (datetime_id = 32)"
"Total runtime: 6510.000 ms"

-- и то же самое для 10 записей:
"Sort  (cost=947.49..947.49 rows=2 width=14) (actual time=15522.000..15522.000 rows=87 loops=1)"
"  Sort Key: event_datetime"
"  ->  HashAggregate  (cost=947.38..947.48 rows=2 width=14) (actual time=15522.000..15522.000 rows=87 loops=1)"
"        ->  Index Scan using period_avg_params_dt_fkey_index on period_avg_params  (cost=0.00..939.32 rows=293 width=14) (actual time=0.000..430.000 rows=46980 loops=1)"
"              Index Cond: (datetime_id = 32)"
"Total runtime: 15522.000 ms"