Sink plughole has small hole inside drain pipe - is this normal?

[mameha]

Bathroom sink has a small hole about 5mm on the side of the drainpipe beneath the plughole.

I checked the other sink in the ensuite, and it has a similar hole.

This seems strange, I cant imagine why this hole would intentionally be there but it looks like it is there by design rather than by damage.

Is this normal as I am tempted to fill it with silicone sealant.

 

[mameha]

like this, which has two holes in the side beneath the plug hole:

 

[bernardgreen]

These holes are how the water from an integrated over flow gets to the drain pipe.

 

[mameha]

eureka - its to bring in water from the overflow hole above the sink... thanks this makes sense.