[SQL] Difference between "foo is false" and "foo=false"? Partial index on boolean.

[Bryce Nesbitt]

Expecting to save 4 seconds per query, I built a partial index on a table, and was surprised that it did not work.  Could someone explain the difference between "foo=false" and "foo is false", for a boolean type column? stage=# create index eg_ve_reconciled_partial on eg_vehicle_event (reconciled) where reconciled=false; stage=# select pg_total_relation_size('eg_ve_reconciled_partial');  pg_total_relation_size ------------------------                    8192 stage=# explain select count(*) from EG_VEHICLE_EVENT where reconciled is false; --------------------------------------------------------------------------  Aggregate  (cost=33169.57..33169.58 rows=1 width=0)    ->  Seq Scan on eg_vehicle_event  (cost=0.00..33169.57 rows=1 width=0)          Filter: (reconciled IS FALSE) stage=# explain select count(*) from EG_VEHICLE_EVENT where reconciled=false; -------------------------------------------------------------------------------------------------------  Aggregate  (cost=1.02..1.03 rows=1 width=0)    ->  Index Scan using eg_ve_reconciled_partial on eg_vehicle_event  (cost=0.00..1.01 rows=1 width=0)          Index Cond: (reconciled = false) The problem is that my test query above is fast, but the real query from Hibernate is still dog slow.  Here's the pg_log entry: LOG:  duration: 4260.575 ms  statement: EXECUTE C_50292  [PREPARE:  select count(vehicleeve0_.VEHICLE_EVENT_ID) as x0_0_ from EG_VEHICLE_EVENT vehicleeve0_ where (vehicleeve0_.CSO_ID=$1 ) and (vehicleeve0_.RECONCILED=$2 )] I tried building two indexes, one for "is false" one for "=false", but the Hibernate query is still slow.  Yet the hand-run version  uses the index easily: stage=# explain analyze select count(vehicleeve0_.VEHICLE_EVENT_ID) as x0_0_ from EG_VEHICLE_EVENT vehicleeve0_ where (vehicleeve0_.CSO_ID=2 )and(vehicleeve0_.RECONCILED=false);                                                                               QUERY PLAN                                                                               -----------------------------------------------------------------------------------------------------------------------------------------------------------------------  Aggregate  (cost=81.75..81.76 rows=1 width=4) (actual time=56.153..56.154 rows=1 loops=1)    ->  Index Scan using eg_ve_reconciled_partial on eg_vehicle_event vehicleeve0_  (cost=0.00..60.05 rows=8679 width=4) (actual time=0.126..44.548 rows=10345 loops=1)          Index Cond: (reconciled = false)          Filter: (cso_id = 2)  Total runtime: 64.825 ms (5 rows) -- ---- Visit http://www.obviously.com/