PostgreSQL and ON expressions

I’ve stumbled upon this shortcoming of PostgreSQL optimizer: the restrictions in the LEFT JOIN’s ON expression are not used to access the outer table. The testcase is fairly basic:

test=# explain 
test-# select * 
test-# from t1 left join t2 on (t1.key1<100 AND t2.key1=t1.col);
                               QUERY PLAN                               
 Hash Left Join  (cost=30832.00..73228.00 rows=1000000 width=16)
   Hash Cond: (t1.col = t2.key1)
   Join Filter: (t1.key1 < 100)
   ->  Seq Scan on t1  (cost=0.00..14425.00 rows=1000000 width=8)
   ->  Hash  (cost=14425.00..14425.00 rows=1000000 width=8)
         ->  Seq Scan on t2  (cost=0.00..14425.00 rows=1000000 width=8)
(6 rows)

The optimizer prefers to use Seq Scan to read 1M rows from table t1, despite the condition “t1.key1<100” which matches only 100 rows, and has a suitable index. If I change LEFT JOIN into INNER, the optimizer will use an index:

test=# explain
test-# select * 
test-# from t1 inner join t2 on (t1.key1<100 and t2.key1=t1.col);
                                 QUERY PLAN                                  
 Nested Loop  (cost=5.59..1136.23 rows=95 width=16)
   ->  Bitmap Heap Scan on t1  (cost=5.16..341.25 rows=95 width=8)
         Recheck Cond: (key1 < 100)
         ->  Bitmap Index Scan on t1_key1  (cost=0.00..5.14 rows=95 width=0)
               Index Cond: (key1 < 100)
   ->  Index Scan using t2_key1 on t2  (cost=0.42..8.36 rows=1 width=8)
         Index Cond: (key1 = t1.col)

The script to reproduce is here.

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