Re: Btree Index on PostgreSQL and Wiredtiger (MongoDB3.2)

>>>>> "Jeff" == Jeff Janes <jeff(dot)janes(at)gmail(dot)com> writes:

Jeff> But shouldn't that still leave us with a 75% full index, rather
Jeff> than slightly over 50% full?

Average is usually about 67%-70%. (For capacity estimation I always
assume 66% for a non-sequentially-filled btree.)

Jeff> The leaf pages start at 50%, grow to 100%, then split back to
Jeff> 50%, then grow back to 100%. So the average should be about 75%.

No, because as the pages split, they fill more slowly (because there are
now more pages). So on average in a large randomly filled index, pages
spend more time nearer 50% full than 100% full. This is easy to
demonstrate by creating a table with an indexed float8 column and adding
batches of random() values to it, checking with pgstatindex at intervals -
the average leaf density will rarely exceed 70%.

However, worst case conditions can give lower leaf densities; obviously
the worst case is if the data is loaded in an order and quantity that
just happens to leave every leaf page recently split.

--
Andrew (irc:RhodiumToad)