Why exists is better than in

Customer Table. Accounts Table. Next Goal Seek in Excel. Article Contributed By :. Easy Normal Medium Hard Expert. Writing code in comment? Please use ide. Load Comments. What's New. Most popular in SQL. TRUE 3! May 20, - am UTC.

October 02, - pm UTC. Could you help with this query, how to evalue it? Hello Tom Thanks for clearing my doubts about the not in. I have following query , could you please help how conceptually evaluate this query?

I am not sure how those two query are executed, Most appreciated if you can give an explaination. Hi Tom Thanks for your explaination about the sql execution. Or is there a universal rule for oracle to feed back the result set, feed back during the sql execution or after the execution? October 09, - am UTC. So, does Oracle have to finish the hash join for all rows? Tuning vikram, November 12, - am UTC. November 13, - pm UTC. May 06, - am UTC. Now, if that row exists - then "select x from t where exists that subquery we know returns AT LEAST one row " - would return every row from T - since the subquery by our definition here returns at least one row.

My question is did CBO treat both the queries as similar? July 06, - pm UTC. Description,rico gjh. Thanks, Vicky. March 15, - am UTC. In order to "tune a query", one needs: o intimate knowledge of the schema - all of the constraints, everything. Hi Tom, Please explain me what is wrong in performance in this query and how to tune this. Thanks in advance.. If the optimizer is unable to merge the correlated subquery into the update - it might have to execute it at least once per row in the update.

If the distinct is necessary - there is something really wrong with your data model, in a big way. A reader, March 21, - am UTC. In this case the account acc is the remote table which is having more than 30millions of rec.

So Now how I will use this hint or is there any alternate to improve the performance? Will appreciate an earliest reply. Thanks Prince. March 21, - am UTC. What could be the reason for this?

Therefore, it is "unknown" whether '2' is not in NULL. With in - we can "know" where '2' in '1', '2', null we Know 2 is in that set. It is by definition the way it works. Part of SQL. A reader, October 24, - am UTC. Many thanks. October 24, - am UTC. Exists was introduced in SQL - the language specification. It is not an Oracle thing - it is a sql thing. It is primarily syntactic convenience, semantic pleasantness. Sometimes "IN" is just "better sounding", sometimes exists is.

In and Exists can be used interchangeable. Not In and Not Exists cannot - they are different regarding treatment of nulls. In and exists both exist for the same reason "for" and "while" loops exist even with goto which can be used instead of for or while as well We can use Scalar Subqueries wherever we can use single row subquery with single column and there are NO any differences between Scalar subquery and Single row retun subquery with single coloumn.

Could you please clarify if the above correct? October 25, - am UTC. You can use scalar subqueries anywhere you can use an expression. We can and will do view merging when we find it to be the appropriate approach. Never assume some order of operation outside of expressions with 's.

Subqueries, inline views, etc etc etc - all can be moved around, rewritten and so on. To see "t. If b has more than one value, that query returns more than 1 rows and will fail in this context.

You need a scalar subquery here! Now, if you wrote instead; where t. A reader, October 25, - am UTC. Why - is it because of the operator 'IN'?

Can u explain the diff between the Output of these queries?? November 16, - am UTC. IF t2. A reader, November 16, - pm UTC. November 16, - pm UTC. If today - b has only one value - but tomorrow it might have more than one - it would not work. Therefore, for this query to work, B must be unique in S. November 17, - pm UTC. Hi Tom, First of all, thank you very much for spending your valuable time in answering all our questions.

I have a question regarding not in vs not exists. May 02, - pm UTC. Hi Tom, I have been trying to fetch rows from table A if it doesn't exist multiple times in table B..

ID ,A. However,I don't get the required output. ID, A. Kindly help me on this. July 30, - am UTC. Hi Tom, i have faced with the strange query execution behavior. In brief, I has two tables. Or simply a bug. Thank you in advance. August 17, - pm UTC. Balaji, August 14, - am UTC. I don't know where to start. I don't know what you mean by "it will not consider NULL records" - but I have a feeling that whatever you would say it means is probably wrong. Not in and VERY null sensitive.

Jess, August 21, - am UTC. Hi Tom, What about 'in' vs. I have a query that looks like this: select txn. The 'status' column in the latter has 3 values and a bitmap index on it. I need to return all transaction with attributes that have any of 2 values.

Run as it is attr. But the plans appear virtually identical in terms of cost. I tried it with a larger number of values in the 'in' list against another table, but that didn't seem to affect the plan either. I would've thought that saying 'not A' would yield better performance than listing 'B, C, D, E' as possible 'in' values Thanks as always. August 28, - pm UTC. The cost if retrieving the same amount of data should be It just means that to get 18,, rows from a table - most of the work, the time, the energy is going to be spent doing IO.

Jess, August 29, - pm UTC. Thanks Tom. I suppose I was expecting the plans to be different erroneously so in a sense that one would be better than the other one, which was not the case. August 29, - pm UTC. In general,! That's a really neat piece of information. I didn't know this. Though I am not sure why that would make the difference. Do you see any reason? December 20, - pm UTC. Follow up to my previous post; Somewhere I read, use of multiple EXISTS with OR clause like below will not execute subsequent clause and that may vary if a column used in the predicate is indexed.

That's exactly the case I tested, but WHY? Why the standalone index makes the query behave differently? Tom - Thanks for your response. I had my peers and DBA looked into it and came to the similar conclusion that it is a bug.

January 02, - am UTC. You just wasted your time and my time by having me look at a fake example that does not represent what you really are faced with. Now we have no idea if you are facing a bug or not - because we have never seen the actual query you are running. The union all might not be giving you the right answer by the way. If more than one branch of the OR is satisfied - you are going to get the same row more than once with a union all.

Rahul, December 29, - am UTC. Hi Tom , Your link provided for IN and exists is awesome. So how does it work? Since we arent matching any values, i suppose the index wont be used. January 04, - am UTC.

We'd use an outer join and keep only the rows where the joined to table was "null" not there at all. Tom - I apologize for not posting the actual query. This solution was found as hit and trial that I don't see the logical reasoning and also impacted the performance unacceptably. Though I don't see the point as you said "The union all might not be giving you the right answer by the way Can you please elaborate why would top query returns multiple records if the EXIST satisfies with multiple clauses.

What am I missing? Hi Tom, I have following queries with execution plans. Both return exact same data. First query uses normal joins, it shows execution plan with low cost and executes faster. I am trying to understand reason for the difference between the execution plans of these 2 queries.

February 04, - am UTC. Anyone that programmed in the 's knows - it takes four characters for years. Always and forever. I'm not a fan of hints at all - so no, I'm not going to give a magical hint - you might try using dynamic sampling set to level 3 or above - the exists query is getting what looks like an incorrect cardinality estimate. I cannot tell really where it is incorrect because you know your data, I do not - you know what indexes go with what tables - i do not, makes it really hard to read a plan I don't know your constraints primary keys, foreign keys, not null, etc and hence cannot even tell you if these two queries are interchangeable!

Dear tom Here tried the following test on Oracle Dear Tom, My bad! Hi tom, I read most of the discussions above. I just want to confirm one thing. If we are dealing with columns which do not have null values.. Thanks in advance Big fan :. February 25, - am UTC.

I mean to say how oracle skip the values "1" and "2" in case of "in" which is not correct as we know null cant be compared with anything null is unknown , how it calculates that "1" in "null" and "2" in "null" are false. Many people say that, that is side effect of "not in" , but could you please say something :.

Hi could you please sujjest me how exactly exit will be used. Used smaple HR schema. Suppose customer is my table in that I want to find all the c ustomers where we don't have their phone numbers.

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