Michael Stone wrote:
> On Thu, Mar 22, 2007 at 01:30:35PM +0200, ismo.tuononen@solenovo.fi wrote:
>> approximated count?????
>>
>> why? who would need it? where you can use it?
>
> Do a google query. Look at the top of the page, where it says "results N
> to M of about O". For user interfaces (which is where a lot of this
> count(*) stuff comes from) you quite likely don't care about the exact
> count...
Right on, Michael.
One of our biggest single problems is this very thing. It's not a Postgres problem specifically, but more embedded in
theidea of a relational database: There are no "job status" or "rough estimate of results" or "give me part of the
answer"features that are critical to many real applications.
In our case (for a variety of reasons, but this one is critical), we actually can't use Postgres indexing at all -- we
wrotean entirely separate indexing system for our data, one that has the following properties:
1. It can give out "pages" of information (i.e. "rows 50-60") without
rescanning the skipped pages the way "limit/offset" would.
2. It can give accurate estimates of the total rows that will be returned.
3. It can accurately estimate the time it will take.
For our primary business-critical data, Postgres is merely a storage system, not a search system, because we have to do
the"heavy lifting" in our own code. (To be fair, there is no relational database that can handle our data.)
Many or most web-based search engines face these exact problems.
Craig
