Query Plan Before GROUP BY

before.sql

SELECT
	"Interval".end_time,
	"MeterData".meter_name,
	"Reading".channel
FROM
	"MeterData"
INNER JOIN "IntervalReadData" ON "MeterData".meter_data_id = "IntervalReadData".meter_data_id
INNER JOIN "Interval" ON "IntervalReadData".interval_read_data_id = "Interval".interval_read_data_id
INNER JOIN "Reading" ON "Interval".interval_id = "Reading".interval_id
 
-- Query Plan:
 
Hash Join  (cost=399095.74..1284498.39 rows=17154293 width=19)
  Hash Cond: ("Reading".interval_id = "Interval".interval_id)
  ->  Seq Scan on "Reading"  (cost=0.00..307110.93 rows=17154293 width=10)
  ->  Hash  (cost=268306.51..268306.51 rows=6764018 width=25)
        ->  Hash Join  (cost=567.97..268306.51 rows=6764018 width=25)
              Hash Cond: ("Interval".interval_read_data_id = "IntervalReadData".interval_read_data_id)
              ->  Seq Scan on "Interval"  (cost=0.00..132458.18 rows=6764018 width=24)
              ->  Hash  (cost=487.42..487.42 rows=6444 width=17)
                    ->  Hash Join  (cost=235.10..487.42 rows=6444 width=17)
                          Hash Cond: ("IntervalReadData".meter_data_id = "MeterData".meter_data_id)
                          ->  Seq Scan on "IntervalReadData"  (cost=0.00..123.44 rows=6444 width=16)
                          ->  Hash  (cost=154.49..154.49 rows=6449 width=17)
                                ->  Seq Scan on "MeterData"  (cost=0.00..154.49 rows=6449 width=17)