Re: Why does using two indexes slow down my query?

The reason for the slowdown is probably:

a)	Counting on condition (1) uses access to the index only.
	total physical reads = a few leaf blocks for 9,000 rows
b)	Counting on condition (2) uses access to the index only
	total physical reads = a few dozen leaf blocks for 80,000 rows
c)	Counting on both conditions requires access to table rows
	9,000 rows to read, on a poor sample space is 1 row in ca. 550
	which might be about 1 block in 8 say.  If your
db_file_multiblock_read_count
	is set to 32 Oracle will choose between:
		using 1 read for every 8th block 
	or	using 1 read to scan 32 blocks (and getting 4 relevant blocks)
	A tablescan is 'clearly' the better option.

	The two pieces of advice you have been given in other posts are both
valid.
	The two-column index is more likely to work for this specific query,
but
	getting better stats is generically a better approach.