Can't it just scan the index to get that? I assumed the index had links to every fileid in the table. In my over-simplified imagination, the table looks like this:
ctid|fileid|column|column|column|column
ctid|fileid|column|column|column|column
ctid|fileid|column|column|column|column
ctid|fileid|column|column|column|column
etc.
While the index looks like
fileid|ctid
fileid|ctid
fileid|ctid
fileid|ctid
...
So I expected scanning the index was faster, and still had everything it needed to do the count. Or perhaps it was because I said COUNT(*) so it needs to look at the other columns in the table? I really just wanted the number of "hits" not the number of records with distinct values or anything like that. My understanding was that COUNT(*) did that, and didn't really look at the columns themselves.
Adrian Klaver wrote:
-------------- Original message ----------------------
From: William Garrison <postgres@mobydisk.com>
I am looking for records with duplicate keys, so I am running this query:
SELECT fileid, COUNT(*)
FROM file
GROUP BY fileid
HAVING COUNT(*)>1
The table has an index on fileid (non-unique index) so I am surprised
that postgres is doing a table scan. This database is >15GB, and there
are a number of fairly large string columns in the table. I am very
surprised that scanning the index is not faster than scanning the
table. Any thoughts on that? Is scanning the table faster than
scanning the index? Is there a reason that it needs anything other than
the index?
I may be missing something, but it would have to scan the entire table to get all the occurrences of each fileid in order to do the count(*).
--
Adrian Klaver
aklaver@comcast.net
