The output of EXPLAIN PLAN is a debug output from Oracle’s query optimiser. The COST is the final output of the Cost-based optimiser (CBO), the purpose of which is to select which of the many different possible plans should be used to run the query. The CBO calculates a relative Cost for each plan, then picks the plan with the lowest cost.
(Note: in some cases the CBO does not have enough time to evaluate every possible plan; in these cases it just picks the plan with the lowest cost found so far)
In general, one of the biggest contributors to a slow query is the number of rows read to service the query (blocks, to be more precise), so the cost will be based in part on the number of rows the optimiser estimates will need to be read.
For example, lets say you have the following query:
SELECT emp_id FROM employees WHERE months_of_service = 6;
months_of_service column has a NOT NULL constraint on it and an ordinary index on it.)
There are two basic plans the optimiser might choose here: Plan 1. Read all the rows from the “employees” table, for each, check if the predicate is true (
months_of_service=6). Plan 2. Read the index where
months_of_service=6 (this results in a set of ROWIDs), then access the table based on the ROWIDs returned.
Let’s imagine the “employees” table has 1,000,000 (1 million) rows. Let’s further imagine that the values for months_of_service range from 1 to 12 and are fairly evenly distributed for some reason.
The cost of Plan 1, which involves a FULL SCAN, will be the cost of reading all the rows in the employees table, which is approximately equal to 1,000,000; but since Oracle will often be able to read the blocks using multi-block reads, the actual cost will be lower (depending on how your database is set up) – e.g. let’s imagine the multi-block read count is 10 – the calculated cost of the full scan will be 1,000,000 / 10; Overal cost = 100,000.
The cost of Plan 2, which involves an INDEX RANGE SCAN and a table lookup by ROWID, will be the cost of scanning the index, plus the cost of accessing the table by ROWID. I won’t go into how index range scans are costed but let’s imagine the cost of the index range scan is 1 per row; we expect to find a match in 1 out of 12 cases, so the cost of the index scan is 1,000,000 / 12 = 83,333; plus the cost of accessing the table (assume 1 block read per access, we can’t use multi-block reads here) = 83,333; Overall cost = 166,666.
As you can see, the cost of Plan 1 (full scan) is LESS than the cost of Plan 2 (index scan + access by rowid) – which means the CBO would choose the FULL scan.
If the assumptions made here by the optimiser are true, then in fact Plan 1 will be preferable and much more efficient than Plan 2 – which disproves the myth that FULL scans are “always bad”.
The results would be quite different if the optimiser goal was FIRST_ROWS(n) instead of ALL_ROWS – in which case the optimiser would favour Plan 2 because it will often return the first few rows quicker, at the cost of being less efficient for the entire query.