sql: OR optimization seems strange or nonexistent

[opomuc]

box.sql.execute("create table t1(id int primary key, a int, b int, c int)")
box.sql.execute("create index i2 on t1(b)")
box.sql.execute("create index i3 on t1(c)")

for i=1,1000000 do
   box.space.T1:insert({i,10*i,i+1,i+100})
end


unix/:/var/run/tarantool/tarantool.sock> test = function(str)
                                       >    local start = clock.monotonic()
                                       >    a = box.sql.execute(str)
                                       >    return clock.monotonic() - start
                                       > end

test("select a,b,c from t1 where a > 10000 or b < 100 or c > 1000000")
---
- 0.29623069998343
...
box.sql.execute("explain query plan select a,b,c from t1 where a > 10000 or b < 100 or c > 1000000")
---
- - [0, 0, 0, 'SCAN TABLE T1']
...
box.sql.execute("create index i4 on t1(a)")
---
...

test("select a,b,c from t1 where a > 10000 or b < 100 or c > 1000000")
---
- 1.487278399989
...
box.sql.execute("explain query plan select a,b,c from t1 where a > 10000 or b < 100 or c > 1000000")
---
- - [0, 0, 0, 'SEARCH TABLE T1 USING COVERING INDEX I4 (A>?)']
  - [0, 0, 0, 'SEARCH TABLE T1 USING COVERING INDEX I2 (B<?)']
  - [0, 0, 0, 'SEARCH TABLE T1 USING COVERING INDEX I3 (C>?)']
...

Execution with all covering indexes lasts ~3 times more than using fullscan