Updating wow account

07 Dec

Although the optimiser is very reliable on simple queries such as this, the real world contains much more complex queries that are beyond the capability of the CBO to optimise.

In some cases the CBO will wrongly believe that a high-volume join is actually low-volume, and wrongly choose a Nested Loops join.

Consider a sub-optimal SQL (SQL1) that takes 10 minutes to run and its optimal tuned equivalent (SQL2) that takes 1 minute to run.

They perform the same logic and return the same results, but one is faster than the other.

Although I haven't performed a study, logic tells me the technique will be slower. So, it makes sense if you only need to go back for one or two more things, but not more. Not at the supermarket with groceries; but in Oracle with data.

It's hardly surprising that you don't see people doing it, right? The database equivalent of running back and forth to the checkout is called Nested I/O.

SELECT empno , ename , dname FROM emp JOIN dept USING (deptno) call count cpu elapsed disk query current rows ------- ------ -------- ---------- ---------- ---------- ---------- ---------- Parse 1 0.00 0.00 0 0 0 0 Execute 1 0.00 0.00 0 0 0 0 Fetch 2 0.00 0.00 0 11 0 14 ------- ------ -------- ---------- ---------- ---------- ---------- ---------- total 4 0.00 0.00 0 11 0 14 Misses in library cache during parse: 1 Optimizer mode: ALL_ROWS Parsing user id: 88 Rows Row Source Operation ------- --------------------------------------------------- 14 MERGE JOIN (cr=11 pr=0 pw=0 time=0 us cost=6 size=364 card=14) 4 TABLE ACCESS BY INDEX ROWID DEPT (cr=4 pr=0 pw=0 time=9 us cost=2 size=52 card=4) 4 INDEX FULL SCAN SYS_C009725 (cr=2 pr=0 pw=0 time=4 us cost=1 size=0 card=4)(object id 70296) 14 SORT JOIN (cr=7 pr=0 pw=0 time=2 us cost=4 size=182 card=14) 14 TABLE ACCESS FULL EMP (cr=7 pr=0 pw=0 time=1 us cost=3 size=182 card=14) This was actually a bit unexpected.

updating wow account-49

Log into your account, select “My Profile”, then click on “Delete Profile” Warning! All your information will be removed and you will be withdrawn from any jobs you applied to.The Rule Based Optimiser is stupid however; one of its rules is to use an index if it is available.This problem will fix itself when we remove the hint.SELECT /* RULE*/ empno , ename , dname FROM emp JOIN dept USING (deptno) call count cpu elapsed disk query current rows ------- ------ -------- ---------- ---------- ---------- ---------- ---------- Parse 1 0.00 0.00 0 0 0 0 Execute 1 0.00 0.00 0 0 0 0 Fetch 2 0.00 0.00 0 26 0 14 ------- ------ -------- ---------- ---------- ---------- ---------- ---------- total 4 0.00 0.00 0 26 0 14 Misses in library cache during parse: 1 Optimizer mode: RULE Parsing user id: 88 Rows Row Source Operation ------- --------------------------------------------------- 14 NESTED LOOPS (cr=26 pr=0 pw=0 time=0 us) 14 NESTED LOOPS (cr=12 pr=0 pw=0 time=15 us) 14 TABLE ACCESS FULL EMP (cr=8 pr=0 pw=0 time=5 us) 14 INDEX UNIQUE SCAN SYS_C009725 (cr=4 pr=0 pw=0 time=0 us)(object id 70296) 14 TABLE ACCESS BY INDEX ROWID DEPT (cr=14 pr=0 pw=0 time=0 us) hint in the above SQL.The Cost Based Optimiser is not stupid; it knows that nested loops is a wasteful plan for this SQL.There can only be one reason why SQL1 is slower: because it is doing That's a total of 4 blocks to process a single sales transaction.Over 1,000,000 sales transactions, that's 4,000,000 block reads just to pick up the customer.Consider that the CUSTOMER table may fit perhaps 50 customers in a single block (probably more, but let's err on the conservative side).This means that the table occupies no more than 200 blocks in total.When joining two tables, for each row in one table Oracle looks up the matching rows in the other table.Consider the following SQL Trace output from TKProf.