pgsql-server: New two-stage sampling method for ANALYZE, as per

Log Message:
-----------
New two-stage sampling method for ANALYZE, as per discussions a few weeks
ago. This should give significantly better results when the density of
live tuples is not uniform throughout a table. Manfred Koizar, with
minor kibitzing from Tom Lane.

Modified Files:
--------------
pgsql-server/src/backend/commands:
analyze.c (r1.71 -> r1.72)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/analyze.c.diff?r1=1.71&r2=1.72)