When you want to write to SDE you need the Esri Geodatabase (ArcSDE Geodb) writer.

I want to write directly to postgresql. I am wondering  posibility..

I have not tried but I guess it is technically possible. What is the reason you do not want to use the SDE reader / writer?

postgresql’s writing performans better than the sde writer. Especially for bulk update. 
I am using the postgresql for attribute update. But what I need to update geometry&attribute, it is updating very slowly.

Than I am wondering posssibility directly using the postgresql db.

I think this can be split in 2 parts:

• The how to technically. I have never tried updating the geometry column, but if I google a bit it looks like it is doable.
• The SDE integrety. You probably need to update the spatial index and the spatial extent of the featureclass, as this is done by SDE when you do a update using the SDE writer.

I’d be really cautious about writing to the underlying SDE database. There are a whole heap of additional steps SDE undertakes (especially if the featureclass is versioned and/or archived). As @nielsgerrits alluded to you’ll need to update indices. I also think there is a separate table that keeps track of objectids