Should avoid hash table creation in monotonic DatetimeIndex?
In very large time series, a first call to

```
ts[stamp]
```

will cause a hash table population to look up the location of `stamp`. If the index has, say, 10mm timestamps, the allocated hash table will take up at minimum 160 megabytes ((timestamp, location) pairs, each 8 bytes) probably rounded up to 256 megabytes. I would say either always use binary search or don't create the hash table when the index exceeds a certain length. 
