The subquery for the derived table t1 can not be merged because it has a GROUP BY clause. Note that alias must be used to distinguish table names in the SQL query that contains correlated subqueries. The query optimizer is more mature for joins than for subqueries, so in many cases a statement that uses a subquery should normally be rephrased as a join to gain the extra speed in performance. subquery to modify a single table is that the optimizer does For a discussion of restrictions on subquery use, including performance issues for certain forms of subquery syntax, see Section 13.2.10.12, “Restrictions on Subqueries”. Interesting. We’re adding work to the optimizer, requiring it to unpack the, admittedly, silly query written above. Yes. I firmly believe in the old adage; if you ain’t cheatin’, you ain’t fightin’. It’s the best explanation I have for why someone would suggest that a sub-query is flat out wrong and will hurt performance. But first, you need to narrow the problem down to MySQL. in “let’s try it harder” the necessary pre-condition of identical meaning is not fulfilled. We could express a query to bring back a single version of one of the documents in one of three ways from the article. Both queries return exactly the same result set. It’s time to put the boot in. join rather than a subquery. Visual EXPLAIN shows that only one of the subqueries are actually merged: Query Plan in MySQL 5.7. You can use the comparison operators, such as >, <, or =. The event takes place before SQLSaturday Providence in Rhode Island, December 2016, therefore, if you’re interested, sign up here. That’s the primary point. <>ALL) subqueries, the optimizer Microsoft has a definition and examples of what a sub-query is right in the MSDN documentation. Twice in the conclusion I also say that you can screw these things up, and you can. Are there situations where a sub-query, of any type, performs perfectly fine, possibly even better than some other construct within SQL Server? All subquery forms and operations that the SQL standard requires are supported, as well as a few features that are MySQL-specific. MySQL Performance Schema. Yes. It’s down to your code and your structure, not simply a single method within the code or structure. MySQL Galera Cluster 4.0 is the new kid on the database block with very interesting new features. One query contains a ranking function (Row_number()) ; the other does not. I need to fill all the results into dataset or datareader with in seconds (with less time). If the subquery actually can be rewritten as a join (most can, but many can't, particularly EXISTS or subqueries that produce aggregates), it typically will be better performing in MySQL. I still prefer JOINs, but I use a lot of sub-queries, including within JOINs. Thanks & Regords Jayaram I used a single example to illustrate the point here. A Subquery is a SELECT statement that is embedded in a clause of another SQL statement. If you’re just seeing completely unsupported, wildly egregious statements, they’re probably not true. not use semijoin or materialization subquery optimizations. OR In fact, Gail Shaw has a blog post or two where she addresses the idea that they automatically lead to a cursor type of situation (which isn’t true). Connectors and APIs. A subquery is a SELECT statement within another statement. In MySQL 5.7 the optimizer will try to merge derived tables into the outer query block. Most often, the subquery will be found in the WHERE clause. If you’re finding any of this useful and you’d like to dig down a little more, you can, because I’ll be putting on an all day seminar on execution plans and query tuning. In MySQL, a subquery is also called an INNER QUERY or INNER SELECT. As with most objects in T-SQL, you can write them horribly, or you can write them well. The things you read on the internet, for example, “don’t use a sub-query because that hurts performance.”. DELETE statements that use a WHERE clause. I think your point is…nothing is inherently bad about subqueries, but your post title leads me to think there is nothing EVER wrong with subqueries. They see one issue, one time, and consequently extrapolate that to all issues, all the time. evaluate subqueries: For IN (or =ANY) However, in some cases, converting a subquery to a join may improve performance. Write a query to find the name (first_name, last_name) and the salary of the employees who have … Sorry, your blog cannot share posts by email. A MySQL subquery is called an inner query while the query that contains the subquery is called an outer query. Second full paragraph said, “…there’s nothing magically good…” and “… “write a sub-query that performs horribly…”. And we all know that there are cases where *it depends* is a valid answer. However, you both seem to agree with me, and disagree with the goofy original premise, a sub-query, in and of itself, is not problematic. rewriting an IN subquery into a join may result in worse performance. Now, this is where things get a little bit interesting. Though subqueries have unique abilities, there are times when it is better to use other SQL constructs such as joins. Here is an example of a subquery: SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2); I prefer the sub-queries because they’re far more readable — and manageable — to me than JOINS. Now here is the script to create the database, tables and insert the values: In this tutorial, you’ll learn how to improve MYSQL performance. The information in them is both wrong and horribly incomplete. In other words, for these plans, everything except the properties of the SELECT operator are exactly the same. They return identical data sets, so they can be compared. Let me put a caveat up front (which I will reiterate in the conclusion, just so we’re clear), there’s nothing magically good about sub-queries just like there is n… [18 Dec 2006 5:47] Ashleigh Gordon In this case I add 1 000 000 records in table1 and table2, just for fun and to show the consequences. A MySQL subquery is a query nested within another query such as SELECT, INSERT, UPDATE or DELETE. Using correlated subqueries 6. As a *human* I don’t see much of a difference in performance between using a sub-query and an [X] JOIN. The world's most popular open source database, Download The subqueries I usually label as “potentially troublesome” are the correlated sub-queries, especially the ones put in the “SELECT” and not in the “FROM”. I truly don’t know what else I could have said that would let you know that “it depends” is in absolute operation here. How we abuse them could be. MySQL subquery slows down drastically, but they work fine independently. Lets add some data! In some cases, it can also provide big performance boosts if used correctly. Let’s go with much more interesting queries that are more likely to be written than the silly example above. 100%. MySQL sub querying capability is a great tool that can be used to create very powerful and readable queries. As Yep. Well, for every record you add in table1, SQL server has to execute the inner query in a nested loop. I am addressing the bad advice that a sub-query is to be avoided because they will inherently lead to poor performance. But always make use of EXPLAIN and know what's in your Query plan , as small differences in your query … Just be careful with them. Yes. These two queries combined will run faster than the first subquery. We’ll run them thousands of times. A subquery using NOT IN *tends* to be poor performing vs, say, a functionally-equivalent LEFT JOIN…WHERE col IS NULL. Let’s start with the similarities. A subquery is usually added within the WHERE Clause of another SQL SELECT statement. We’re just going to mess with two of them. Displayed in pink are the common sets of operations between the two plans. Let’s add in a statement to free the procedure cache on each run and retry the queries. In MySQL, the main query that contains the subquery is also called the OUTER QUERY or OUTER SELECT. a workaround, try rewriting them as multiple-table These subqueries are also called nested subqueries. They too are subject to the ability of the optimizer to logically deal with them. In short, the optimizer created two identical execution plans. has these choices: For derived tables, the optimizer has these choices: Merge the derived table into the outer query block, Materialize the derived table to an internal temporary table. You could argue that we’re comparing two completely different queries, but that’s not true. In MySQL subquery can be nested inside a SELECT, INSERT, UPDATE, DELETE, SET, or DO statement or inside another subquery. Materialization speeds up query execution by generating a subquery result as a temporary table, normally in memory. Performance tuning MySQL depends on a number of factors. As with any language, even simple commonly used functions can become problematic if poorly coded — or poorly planned. Why do correlated subqueries kill the performance? Any help must be appreciated. What about execution times? There is a measurable difference now: More work is done by the optimizer on the sub-query to compile the same execution plan. When we refer only to the compile time and not the execution time, there is a performance hit. Posted by: Frank Osterberg Date: November 15, 2005 11:19AM ... that to my knowledge means that the problem is not the query but something with MySql and subqueries. It took an extra tic on the CPU and just a little more CompileMemory and CompileTime. People should provide both the queries they are testing with and the numbers that their tests showed. Let’s compare the plans using the new SSMS plan comparison utility: Well, darn. Japanese, 8.2.2.1 Optimizing Subqueries with Semijoin Transformations, 8.2.2.2 Optimizing Subqueries with Materialization, 8.2.2.3 Optimizing Subqueries with the EXISTS Strategy. Just as you can with any kind of query. Using subquery to return a list of values (known as column subquery) 4. Let me put a caveat up front (which I will reiterate in the conclusion, just so we’re clear), there’s nothing magically good about sub-queries just like there is nothing magically evil about sub-queries. They can be very useful to select rows from a table with a condition that depends on the data in the same or another table. You absolutely can. In conclusion, it’s safe to use sub-queries. I was curious, as from my general understanding, there should not be any difference in execution time between sub-query and join, provided the meaning of both queries is the same and provided the optimizer does what it is supposed to do and produces identical query plans. this Manual, Block Nested-Loop and Batched Key Access Joins, Optimizing Subqueries with Semijoin Transformations, Optimizing Subqueries with Materialization, Optimizing Subqueries with the EXISTS Strategy, InnoDB and MyISAM Index Statistics Collection, Optimizing for Character and String Types, Disadvantages of Creating Many Tables in the Same Database, Limits on Table Column Count and Row Size, Optimizing Storage Layout for InnoDB Tables, Optimizing InnoDB Configuration Variables, Optimizing InnoDB for Systems with Many Tables, Caching of Prepared Statements and Stored Programs, Using Symbolic Links for Databases on Unix, Using Symbolic Links for MyISAM Tables on Unix, Using Symbolic Links for Databases on Windows, Measuring the Speed of Expressions and Functions, Measuring Performance with performance_schema, Examining Server Thread (Process) Information, Replication Replica Connection Thread States, MySQL NDB Cluster 7.3 and NDB Cluster 7.4, 8.0 Without the recompile, there is no performance hit. Let’s start with a simple test, just to validate the concept of how a sub-query performs within SQL Server: If there is something inherently wrong with a sub-query, then there is something twice as wrong with two sub-queries. This is why I’ve been writing all these blog posts against the goofy, single-statement, performance check-lists I’m seeing spring up all over the place. Subqueries by themselves are not necessarily bad. As with so many things in SQL Server and T-SQL, it depends. I have used sub-queries similar to the examples above and I have found NO issues with using them on projects that have tens of thousands of rows throughout several tables, with at least 50 columns and some with up to 600 columns (not my design!). But some subqueries are demonstrably bad and people do word association tricks without understanding the subquery antipatterns. Starting with MySQL 4.1, all subquery forms and operations that the SQL standard requires are supported, as well as a few features that are MySQL-specific. I’m not arguing that you can’t screw up your system with poor coding practices. Queries of that form: SELECT * FROM Table1 WHERE col NOT IN (SELECT col Nested SubQuery Performance/Problem. Are there situations where a sub-query, of any type, can lead to poor performance? UPDATE and It's better in MySQL 5.6, but it can still be costly because it tends to run the subquery as a dependent subquery, that is, it executes the subquery once for each distinct value of Table1.col. Ask Question ... you are basically executing a subquery for every row of mybigtable against itself. The optimizer uses materialization to enable more efficient subquery processing. You cheated by using an APPLY function, which I’ve found most people tend not to do. The following discussion provides more information about the You can absolutely write a sub-query that performs horribly, does horrible things, runs badly, and therefore absolutely screws up your system. Currently it is available only as a part of MariaDB 10.4 but in the future it will work as well with MySQL 5.6, 5.7 and 8.0. When I’m looking at your query structure, it seem as if you’re writing a derived table than a sub-query. First of all, we have exactly the same QueryPlanHash value in both plans. Absolutely. To get a flavor of the performance improvements that MariaDB 5.3 delivers compared to MySQL, check our presentation at this years MySQL conference: However, what about that extra little bit of compile time in the query that used sub-queries? Optimize MySQL Performance with Session Variables and Temporary Tables. The average results from the Extended Events sql_batch_completed event were 75.9 microseconds for both queries. The first example, silly but illustrative, shows that there actually is a performance difference as more time is spent compiling the plan. Therefore, the queries you write can be fairly sophisticated before, by nature of that sophistication, you begin to get serious performance degradation. A Subquery is used to return data that will be used in the main query as a condition to further restrict the data to be retrieved. preceding optimization strategies. You need to have a method of validation for some of what you read on the internet. At least in this example. MySQL also lets you create temporary tables with the CREATE TEMPORARY TABLE command. Have to agree with Dave. Also, in the tests, it pretty clearly shows that there is a performance cost during optimization. The differences are visible in the average execution time, about a 20% improvement. In this blog post we would like to go over some of the new features that came along with Galera Cluster 4.0. I double (and triple) checked the definition of what a sub-query is. They are another construct that can be used or abused. Description: Similar to bug 4040, performance with subqueries with group by/having clauses is very slow. For information about how the optimizer handles subqueries, see Section 8.2.2, “Optimizing Subqueries, Derived Tables, and View References”. Whether or not you get a performance hit from a sub-query then, in part, depends on the degree to which you’re experiencing compiles or recompiles. I’m positive that I said, twice, in the post, that there is nothing inherently positive, just as there is nothing inherently negative around sub-queries. I’ve written before about the concept of cargo cult data professionals. The MySQL query optimizer has different strategies available to One that uses a sub-query, and one that does not: As per usual, we can run these once and compare results, but that’s not really meaningful. I do find that using an APPLY operator greatly affects the performance and I don’t tend to have a problem with derived table sub-queries or those used in an APPLY because they’re often quite acceptable and usually better than what was there before. You can see that the regular query is doing a lot more work to arrive at an identical set of data. , the first subquery example is terrible from performance point of View on average, subquery! Optimizer handles subqueries, see Section 8.2.2, “Optimizing subqueries, derived Tables, and consequently extrapolate that all! Mysql sub querying capability is a performance hit silly but illustrative, shows that is. Situations where a sub-query that performs horribly… ” — or poorly planned your and! All, we have exactly the same execution plan that to all,! Take a look at those properties: OK. now we have some interesting similarities inherently lead to performance... 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