are there any methods to disable updating index before inserting large number tuples?

[sunpeng <bluevaley@xxxxxxxxx>]

hi, friends,
  Now each hour I want to insert about 20 millions tuples into table A,  which has a         btree index.
  How to disable index update before inserting those 20 millions tuples, and then enable it after those tuples having being inserted? Just as the followings:
  >disable index update
  >insert into A ....//here will insert 20 millions tuples
  > enable index update

  The purpose is to improve the performance.
  Thanks!
peng