DB2 can satisfy the query using only indexes because DB2 doesn't do MVCC.
Although MVCC is generally a win in terms of making the database easier to use and applications less brittle, it also means that the database must inspect the visibility information for each row before it can answer a query. For most types of queries this isn't a big deal, but for count(*) type queries, it slows things down.
Mark,
Thanks for the answer. My DBAs just got this look on thier face when I showed. It's not like the couldn't have investigated this information themselves but I think the light finally came on.
One question that we came up with is how does this affect other aggregate functions like MAX,MIN,SUM and whatnot? Being that this is our data warehouse, we use these all the time. As I've said previously, I didn't know a human could generate some of the queries we've passed through this system.
Since adding the visibility information to indexes would make them significantly more expensive to use and maintain, it isn't done. Therefore, each row has to be fetched from the main table anyway.
Since in this particular query you are counting all rows of the database, PG must fetch each row from the main table regardless, so the sequential scan is much faster because it avoids traversing the index and performing random read operations.
