Index split WAL reduction

Hi,

Currently, an index split writes all the data on the split page to WAL.
That's a lot of WAL traffic. The tuples that are copied to the right
page need to be WAL logged, but the tuples that stay on the original
page don't.

Here's a patch to do that. It needs further testing, I have used the
attached crude crashtest.sh to test the basics, but we need to test the
more obscure cases like splitting non-leaf or root page.

On a test case that inserts 10000 rows in increasing key order with a
100 characters wide text-field as key, the patch reduced the total
generated WAL traffic from 45MB to 33MB, or ~ 25%. Your mileage may
vary, depending on the tuple and key sizes, and the order of inserts.

Anyone see a problem with this?

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com