Cost – again

Browsing through some postings on Tony Hasler’s blog a little while ago I found this response to a note he had posted on some anomalies (i.e. bugs) in the costing of the “(min/max)” index scans:

My current understanding is it is not valid to try to compare costs across different queries (even if you just alter it by adding a hint). In general a better plan will have a lower cost but you cannot rely on this metric. The metric is really for the CBO to choose between alternative plans for this specific query, not to compare plans generated for different queries.

Now I know that this is a statement that pretty much paraphrases something that Tom Kyte wrote on AskTom several years ago – but it’s wrong. As I’ve pointed out in the past, “Cost is Time”. The cost of a query represents the optimizer’s estimate of how long it will take that query to run – so it is perfectly valid to compare the cost of two queries to see which one the optimizer thinks will be faster but, thanks to limitations and defects in the optimizer it may not be entirely sensible to do so.

The point I want to address in this post though is the comment that “it’s valid to compare the cost of different plans, but not to compare the cost of two different queries”. Consider the following queries:

select 
	t1.v1
from 
	t1
where	t1.id in (
		select
			t2.id 
		from	t2
		where	t2.n1 = 15
	)
;


select 
	t1.v1
from 
	t1
where	exists (
		select
			t2.id 
		from	t2
		where	t2.n1 = 15
		and	t2.id = t1.id
	)
;


select
	t1.v1
from
	(
		select	distinct t2.id
		from	t2
		where	t2.n1 = 15
	) t2,
	t1
where
	t1.id = t2.id
;


select
	v1.v1
from	(
	select
		distinct
			t1.rowid,
			t2.id,
			t1.v1
	from
		t2, t1
	where
		t2.n1 = 15
	and	t1.id = t2.id
	) v1
;

Tables t1 and t2 have the following definitions, so all four queries are logically equivalent:

Name                    Null?    Type
----------------------- -------- ----------------
ID                      NOT NULL NUMBER
N1                      NOT NULL NUMBER
V1                               VARCHAR2(6)
PADDING                          VARCHAR2(100)

According to the claim it is not valid to compare the costs that the optimizer gives you for these four different queries – but they are the same query. In principle the optimizer might transform the IN subquery to an EXISTS subquery, it might simply unnest the subquery, it might unnest the subquery and merge the resulting inline view- so when the optimizer is “comparing different costs for the same query”, it will also be “comparing costs of different queries”.

Cost IS time – but only in theory. The “trick” to sorting out optimization problems lies in recognising where the optimizer model is not right for your data, or the optimizer arithmetic is too simplistic or has a bug. A fact which Tom confirms in the same thread some seven years after the original question.