For the query
Select col1 from table1
Where col1 like '172.%'
The table has 133 million unique ip addresses. Col1 is indexed.
The optimizer is using a sequential scan
This is the explain analyze output
"Seq Scan on table1 (cost=0.00..2529284.80 rows=1 width=15) (actual
time=307591.339..565251.775 rows=524288 loops=1)"
" Filter: ((col1)::text ~~ '172.%'::text)"
"Total runtime: 565501.873 ms"
The number of affected rows (500K) is a small fraction of the total row
count.