What is the significance of "cost" column in explain plan

Question

Hi,
Can anyone explain what is the meaning of the values which we get in the cost column in the explain plan..For Ex : Cost : 4500 . What does this value mean...and is it measured in which form of units...i mean seconds,nanoseconds etc

Answer 1

kingfisher,
Ok one more link for you but I shall quote the text also here,
http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/optimops.htm#i82005
The cost is an estimated value proportional to the expected resource use needed to execute the statement with a particular plan. The optimizer calculates the cost of access paths and join orders based on the estimated computer resources, which includes I/O, CPU, and memory.
And few paragraphs down,
13.4.1.3.3 Cost
The cost represents units of work or resource used. The query optimizer uses disk I/O, CPU usage, and memory usage as units of work. So, the cost used by the query optimizer represents an estimate of the number of disk I/Os and the amount of CPU and memory used in performing an operation. The operation can be scanning a table, accessing rows from a table by using an index, joining two tables together, or sorting a row set. The cost of a query plan is the number of work units that are expected to be incurred when the query is executed and its result produced.
The access path determines the number of units of work required to get data from a base table. The access path can be a table scan, a fast full index scan, or an index scan. During table scan or fast full index scan, multiple blocks are read from the disk in a single I/O operation. Therefore, the cost of a table scan or a fast full index scan depends on the number of blocks to be scanned and the multiblock read count value. The cost of an index scan depends on the levels in the B-tree, the number of index leaf blocks to be scanned, and the number of rows to be fetched using the rowid in the index keys. The cost of fetching rows using rowids depends on the index clustering factor. See "Assessing I/O for Blocks, not Rows".
The join cost represents the combination of the individual access costs of the two row sets being joined, plus the cost of the join operation.
Now I guess if you read this part, it should be pretty clear what cost is.Cost is an evlaution of the resource that is estimated by Oracle for every step incurred in the uery execution.There are no of steps and each may involve doing IO,consuming CPU and/or using memory.Cost is a factor which oracle uses combining all of these 3 (depending on the version) together to propose the work done or expected to be done in exeuting a query.So this actualy should represent the time spent exactly on each and every step.That's what the objective frm cost is.But at the moment,this is not there.There amy be a situation that cost is shown as very high but query is working fine.There are woraround which can bring the cost down for example using and tweaking optimizer_index_cost_adj parameter , we can propose oracle regarding our indexes and it may pick up lesser cost evaluation.What is mentioned is that this model is becoming more and more mature and in the next releases, we may see that cost is representing exactly the time that we would spend inthe query.At the moment,its not there.So as Chris mentioned,Tuning for low cost only is not a good way.
I suggest you grab a copy of JL's book.He has explained it much better in his book.
Hope I said some thing useful.
Aman....