mysql subquery performance

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. 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. 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. 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. 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. At least in this example. Here is an example of a subquery: SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2); What about execution times? I still prefer JOINs, but I use a lot of sub-queries, including within JOINs. However, running a query once or twice isn’t testing. It’s time to put the boot in. MySQL Performance Schema. subquery to modify a single table is that the optimizer does They return identical data sets, so they can be compared. UPDATE and That’s the primary point. We’ll run them thousands of times. Now here is the script to create the database, tables and insert the values: 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. Here are the resulting execution plans: Huh, look sort of, I don’t know, almost identical. Well, for every record you add in table1, SQL server has to execute the inner query in a nested loop. Description: Similar to bug 4040, performance with subqueries with group by/having clauses is very slow. join rather than a subquery. We’re just going to mess with two of them. However, this is not possible for all subqueries. And we all know that there are cases where *it depends* is a valid answer. Are there situations where a sub-query, of any type, can lead to poor performance? Microsoft has a definition and examples of what a sub-query is right in the MSDN documentation. More examples can be made to make the point in the other direction. 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. In addition, a subquery can be nested inside another subquery. As with any language, even simple commonly used functions can become problematic if poorly coded — or poorly planned. Reiterating lest anyone else think I’m unclear. To get a flavor of the performance improvements that MariaDB 5.3 delivers compared to MySQL, check our presentation at this years MySQL conference: Let’s compare the plans using the new SSMS plan comparison utility: Well, darn.  current, 5.6  How to repeat: Please see attached files for example tables and query, taken from bug 4040 . OR 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!). WHERE clause. In Transact-SQL, there is usually no performance difference between a statement that includes a subquery and a semantically equivalent version that does not. Therefore, the queries you write can be fairly sophisticated before, by nature of that sophistication, you begin to get serious performance degradation. So you cannot compare them. If you’re just seeing completely unsupported, wildly egregious statements, they’re probably not true. Lets add some data! I’ve written before about the concept of cargo cult data professionals. The world's most popular open source database, Download If you go and read the article, depending on how the indexes are structured and the amount of data in play, either approach can perform better. 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 Yes. Using EXISTS and NOT EXISTS in correlated subqueries in MySQL 7. Is it possible for you to write horrid code inside of a sub-query that seriously negatively impacts performance? In short, it depends. The things you read on the internet, for example, “don’t use a sub-query because that hurts performance.”. The query optimization process within SQL Server deals well with common coding practices. preceding optimization strategies. The first example, silly but illustrative, shows that there actually is a performance difference as more time is spent compiling the plan. 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. How to increase the execution performance of the subquery?. Both queries return exactly the same result set. We could express a query to bring back a single version of one of the documents in one of three ways from the article. One query contains a ranking function (Row_number()) ; the other does not. rewriting an IN subquery into a join may result in worse performance. Let’s get a few thousand runs of both queries. 100%. Why do correlated subqueries kill the performance? 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… A Subquery is a SELECT statement that is embedded in a clause of another SQL statement. When we refer only to the compile time and not the execution time, there is a performance hit. You need to have a method of validation for some of what you read on the internet. They see one issue, one time, and consequently extrapolate that to all issues, all the time. However, in some cases where existence must be checked, a join yields better performance. The differences are visible in the average execution time, about a 20% improvement. Note that alias must be used to distinguish table names in the SQL query that contains correlated subqueries. Knowing about a subquery versus inner join can help you with interview questions and performance issues. It’s worth noting that correlated sub-queries are frequently more problematic than other types of sub-queries. MySQL also lets you create temporary tables with the CREATE TEMPORARY TABLE command. Sorry, your blog cannot share posts by email. Connectors and APIs. It’s down to your code and your structure, not simply a single method within the code or structure. So, in conclusion then, again, there is nothing inherently problematic about a sub-query, but rather, how it is used. MySQL subquery slows down drastically, but they work fine independently. This rather simple point seems to have ruffled feathers because, yes, there are exceptions. Absolutely. Using subquery to return one ore more rows of values (known as row subquery) 5. 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. The optimizer uses materialization to enable more efficient subquery processing. I am addressing the bad advice that a sub-query is to be avoided because they will inherently lead to poor performance. The information in them is both wrong and horribly incomplete. In MySQL 5.7 the optimizer will try to merge derived tables into the outer query block. [18 Dec 2006 5:47] Ashleigh Gordon Most often, the subquery will be found in the WHERE clause. They see one issue, one time, and consequently extrapolate that to all issues, all the time. How we abuse them could be. In some cases these can be rewritten as LEFT JOINs, in other cases the query optimizer figures it out for you. However, what about that extra little bit of compile time in the query that used sub-queries? SELECT column_name(s) FROM table_name_1 WHERE column_name expression_operator{=,NOT IN,IN, <,>, etc}(SELECT column_name(s) from table_name_2); 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. Visual EXPLAIN shows that only one of the subqueries are actually merged: Query Plan in MySQL 5.7. <>ALL) subqueries, the optimizer 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. With a few runs on average, the execution times were identical at about 149mc with 11 reads. We are using C# with MYSQL database. The first time MySQL needs the subquery result, it materializes that result into a temporary table. Are there situations where a sub-query, of any type, performs perfectly fine, possibly even better than some other construct within SQL Server? 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”. But always make use of EXPLAIN and know what's in your Query plan , as small differences in your query … The results are even more dramatic when we take away the compile time: We can also look to the execution plans to get an understanding of how these queries are being resolved: The plan on top is the sub-query plan, and the plan on the bottom is the plan for just the plain query. As a *human* I don’t see much of a difference in performance between using a sub-query and an [X] JOIN. It took an extra tic on the CPU and just a little more CompileMemory and CompileTime. In conclusion, it’s safe to use sub-queries. The event takes place before SQLSaturday Providence in Rhode Island, December 2016, therefore, if you’re interested, sign up here. In this blog post we would like to go over some of the new features that came along with Galera Cluster 4.0. Yes, correlated sub-queries are one type, but the derived table is also a type of a sub-query. You can absolutely write a sub-query that performs horribly, does horrible things, runs badly, and therefore absolutely screws up your system. You could argue that we’re comparing two completely different queries, but that’s not true. See Section 13.2.10.11, “Rewriting Subqueries as Joins”. Once the query is compiled, the performance is identical. In short, the optimizer created two identical execution plans. MySQL Enterprise Edition. MySQL Galera Cluster 4.0 is the new kid on the database block with very interesting new features. They are another construct that can be used or abused. Practice #1: Using correlated subquery. A limitation on UPDATE and A subquery is usually added within the WHERE Clause of another SQL SELECT statement. Using correlated subqueries 6. In other words, for these plans, everything except the properties of the SELECT operator are exactly the same. Using subquery to return a list of values (known as column subquery) 4. As with so many things in SQL Server and T-SQL, it depends. There is a measurable difference now: More work is done by the optimizer on the sub-query to compile the same execution plan. The MySQL query optimizer has different strategies available to In some cases, it can also provide big performance boosts if used correctly. SELECT DISTINCT t1.column1 FROM t1, t2 WHERE t1.column1 = t2.column1; Some subqueries can be transformed to joins for compatibility with older versions of MySQL that do not support subqueries. We’re adding work to the optimizer, requiring it to unpack the, admittedly, silly query written above. By default, MySQL doesn’t allow us to add a limit clause in our subqueries, when using specific clauses like WHERE IN. MySQL Performance Schema MySQL Replication Using the MySQL Yum Repository MySQL NDB Cluster 8.0. version 8.0 5.7 5.6 ... A subquery is a SELECT statement within another statement. They too are subject to the ability of the optimizer to logically deal with them. In MySQL, a subquery is also called an INNER QUERY or INNER SELECT. You absolutely can. In MySQL, the main query that contains the subquery is also called the OUTER QUERY or OUTER SELECT. Post was not sent - check your email addresses! You can see that the regular query is doing a lot more work to arrive at an identical set of data. A subquery using NOT IN *tends* to be poor performing vs, say, a functionally-equivalent LEFT JOIN…WHERE col IS NULL. See, the optimizer actually worked a little harder to create the first plan than the second. evaluate subqueries: For IN (or =ANY) a workaround, try rewriting them as multiple-table A sub-query typically look like Optimize MySQL Performance with Session Variables and Temporary Tables. Though subqueries have unique abilities, there are times when it is better to use other SQL constructs such as joins. Yes. Have to agree with Dave. Also, to be sure we’re comparing apples to apples, we’ll force a recompile on every run, just like in the first set of tests. I used a single example to illustrate the point here. Subqueries by themselves are not necessarily bad. Second full paragraph said, “…there’s nothing magically good…” and “… “write a sub-query that performs horribly…”. SELECT column FROM table WHERE column = (SELECT xx FROM xx). 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. DELETE statements that use a Just as you can with any kind of query. Let’s assume some versioned data like in this article on Simple-Talk. Yep. You cheated by using an APPLY function, which I’ve found most people tend not to do. It’s the best explanation I have for why someone would suggest that a sub-query is flat out wrong and will hurt performance. MySQL sub querying capability is a great tool that can be used to create very powerful and readable queries. Using subquery in SELECT statement in MySQL 8. Let’s take a look at those properties: OK. Now we have some interesting differences, and especially, some interesting similarities. Rows and costs problem down to your code and your structure, not simply a single method the... A statement to free the procedure cache on each run and retry the queries in worse performance and the that..., again, there ’ s nothing magically good… ” and “ … write... Slows down drastically, but i use a lot more work to arrive at an identical set of data poor! Just fine this article on Simple-Talk issues, all the time not share posts by.. To free the procedure cache on each run and retry the queries silly example above measurable difference now: work. Mysql doesn’t allow us to add a limit clause in our subqueries, when using specific clauses like where.. Article on Simple-Talk EXISTS in correlated subqueries in MySQL, the subquery? s time to put the in! Subqueries in MySQL 5.7 go over some of what you read on the internet tricks without understanding the subquery called. But the derived table t1 can not be merged because it has a definition and examples what. 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