I have a large table (~ 2B rows) that contains an indexed timestamp column. I
am attempting to run a query to determine the number of rows for a given day
using something like "select count(*) from tbl1 where ts between '2008-05-12
00:00:00.000' and '2008-05-12 23:59:59.999'". Explain tells me that the query
will be done using an index scan (as I would expect), and I realize that it is
going to take a while. My question concerns some unusual I/O activity on the
box (SUSE) when I run the query.
For the first couple of minutes I see reads only. After that vmstat shows
mixed reads and writes in a ratio of about 1 block read to 5 blocks written.
We have determined that files in our data and log partitions are being hit, but
the file system itself is not growing during this time (it appears to be
writing over the same chunk of space over and over again). Memory on the box
is not being swapped while all of this is happening. I would have guessed that
a "select count(*)" would not require a bunch of writes, and I can't begin to
figure out why the number of blocks written are so much higher than the blocks
read. If I modify the where clause to only count the rows for a given minute
or two, I see the reads but I never see the unusual write behavior.
Any thoughts into what could be going on? Thanks in advance for your help.
Doug
