Actions are also known as operations. It has been found that by changing the default value of the optimizer_max_permutations setting to a value less than the original setting that join orders are evaluated first. DISTINCT 10. Column order in the SELECT clause or an ON or WHERE clause makes no difference. tables in your query are going to have their join order forced (not evident in this example...but imagine we were joining 4 or 5 tables in total). that I thought would make for a good blog post: ...I've been wondering if it really matters from a performance standpoint where I start my queries. -- A number of rows we know is larger than our table. It's declarative until you care about performance, which given the way SQL queries tend to very easily describe O(n 3), O(n 4), O(n join_tables) algorithms, is generally almost immediately.. This tutorial guides you through main concept of performance with tips and tricks about indexes and when to use them and which columns to choose as indexes. SELECT 9. Does the order of the clauses matter? Perhaps a sample of the two different orders you are talking about. On the other hand, for a given query that uses an index, column order in the index can be very important. Disclaimer: For this post, I'm only going to be talking about INNER joins. This effect is not worth worrying about for only three tables, but it can be a lifesaver with many tables. FROM and JOINs. The key thing to take away and I highly recommend you watch it. I had a great question submitted to me (thank you Brandman!) GROUP BY 6. So if the order that our tables are joined in makes a big difference for performance reasons, SQL Server follows the join order we define right? Dear Tom,Yesterday we had a discussion at lunch regarding the performance impact of how the WHERE clause is constructed. There is two tables named Table-A and In an emergency "production-servers-are-on-fire" scenario, I might use a query or join hint to immediately fix a performance issue and go back to implement a better solution once things calm down. much concerned about  performance. An example of such a "readability" order is mentioned in shop standard example 1 (code join predicates before local predicates). Since the StockItems table has no duplicate rows (it's a simple lookup table for product information) it is a great table to join with as early as possible since it will reduce the total number of rows getting passed around for the remainder of the query. https://www.sqlskills.com/blogs/kimberly/the-accidental-dba-day-15-of-30-statistics-maintenance/). Many operations apply filters, which means that as you build a view and add filters, those filters always execute in the order established by the order of operations. Join the DZone community and get the full member experience. Most of the time you can take advantage of any order that makes the SQL more readable and easier to maintain without affecting performance. Here [Table-A] JOIN [Table-B] or [Table-B] JOIN [Table-A], MS SQL Server knows it well that both are same. The optimizer does not consider join orders that violate this rule. The order in which the tables in your queries are joined can have a dramatic effect on how the query performs. WITH CUBE or WITH ROLLUP 7. It uses a hash table to aid in joining. FROM 2. This tip will look at the order of the columns in your index and how … Over a million developers have joined DZone. In the above Statistics are also a whole 'nother topic for a whole 'nother day (or month) of blog posts, so to not get too side tracked with this post, I'll point you to Kimberly Tripp's introductory blog post on the subject: Since in our example query SQL Server is already joining the tables in the most efficient order, let's force an inefficient join by joining Orders with OrderLines first. check your statistics first So even if we rearrange the order of the tables in our FROM statement like this: Or even if we rewrite the tables into subqueries: SQL Server will interpret and optimize our three separate queries (plus the original one from the top of the page) into the same exact execution plan: Basically, no matter how we try to redefine the order of our tables in the FROM statement, SQL Server will still do what it thinks it's best. “One common question that -- The logical ordering of the tables during an Inner Join -- doesn't matter. Step-1 [ Create Base Table and Insert Some Records ]. JOIN 4. Maybe production has a problem and I need to get things running again; a query or join hint may be the quickest way to fix the immediate issue. Query and join hints will successfully force the order of the table joins in your query, however they have significant draw backs. This is especially true with large and complex queries where knowing the order of execution can save us from unwanted results, and help us create queries that execute faster. https://www.sqlskills.com/blogs/kimberly/the-accidental-dba-day-15-of-30-statistics-maintenance/), Adam Machanic's fantastic presentation on the subject. However, it can be argued that join order is the most important aspect of an execution plan. all This is logical though: not actual. The question was the following:Assuming a variable @var that is an integer and has a value of 0 (zero).What is the best … Winning solutions will be posted on this blog with … Generally speaking this is not the most efficient join type for SQL Server; Loop Join is much … Basically, join order DOES matter No matter how SQL Server actually does it, these semantics are honoured to the … For a hash join to work, at least one of the join conditions will need to be a equijoin, that is, two columns that are equal (=) … Basically, join order DOES matter because if we can join two tables that will reduce the number of rows needed to be processed by subsequent steps, then our performance will improve. Most … HAVING 8. To understand it lets take Table-B. In terms of performance, it's almost certain that the latter scenario (joining OrderLines with StockItems first) will be faster because StockItems will help us be more selective. SQL is a declarative language: you write code that specifies *what* data to get, not *how* to get it. It's up to the Query Optimnizer to arrange -- the tables in the best order. The tables specified in the FROM clause (including JOINs), will be evaluated first, to determine the entire working set which is relevant for the query. So you already checked to see if your statistics are the problem and exhausted all possibilities on that front. all know that whenever a SQL Query is executed the MS SQL server ALTER TABLE Warehouse.StockItems SET (SYSTEM_VERSIONING = OFF); ADD CountryOfManufacture AS CAST(JSON_VALUE(CustomFields,'$.CountryOfManufacture') AS NVARCHAR(10)). It does this by using precalculated statistics on your table sizes and data contents in order to be able to pick a "good enough" plan quickly. This join type is probably the most common one that you will encounter. Oracle Tips by Burleson Consulting October 26, 2009. This is my favorite way of forcing a join order because we get to inject control over the join order of two specific tables in this case (Orders and OrderLines) but SQL Server will still use its own judgement in how any remaining tables should be joined. Most of the time, IN and EXISTS give you the same results with the same performance. The optimizer can choose an index as the access path for a table if it is the inner table, but not if it is the outer table (and there are no further qualifications). It is available in respect of all contracts except positive contracts of a personal nature (e.g. It is not a bad effort related improve the performance of query. As in, if I put the ASI_EVENT_TIME clause first (since that would remove the most of the results out of any of the clauses. By default SQL Server gives you no control over the join order - it uses statistics and the query optimizer to pick what it thinks is a good join order. In the first you are saying INNER JOIN TABLEB B ON B.COLA = A.COLA LEFT OUTER JOIN TABLEC C ON C.COLB = B.COLB AND B.COLC IN ('','Y','O') and in the second INNER JOIN TABLEB B ON B.COLA = A.COLA AND B.COLC IN ('','Y','O') LEFT OUTER JOIN TABLEC C ON C.COLB = B.COLB So, firstly rows are filtered by the join … Its importance is sometimes underestimated and join order is often overlooked when a query needs optimization. create several query plans with different join Order and choose the best With the cost-based approach, the optimizer's choice of join orders can be overridden with the ORDERED hint. -- Run if if you want to follow along - add a computed column and index for CountryOfManufacture. The join works in two phases, the build phase and the probe phase. The same problem exists with using a join hints: Using the LOOP hint successfully forces our join order again, but once again the join order of all of our tables becomes fixed: A join hint is probably the most fragile hint that forces table join order because not only is it forcing the join order, but it's also forcing the algorithm used to perform the join. If your query happens to join all the large tables first and then joins to a smaller table later this can cause a lot of unnecessary processing by the SQL engine. SQL where clause order can change performance. For example, if I join from A-B-C, would I be better off starting at table B and then going to A & C? On the other hand, when you use JOINS you might not get the same result set as in the IN and the EXISTS clauses. TOP A derived table follows this, then the outer query does it again etc etc. The performance will be measured using the Actual Execution Plan and SET IO Statistics ON The result set returned from the query should be the same before changing the order of columns in WHERE condition and after changing order of columns in WHERE condition. We can turn it off using the undocumented query hint Let's look into each of the SQL query parts according to their execution order. Well you might notice that our StockItems table is small with only 227 rows. But if we tell the planner to honor the JOIN order, the second and third take less time to plan than the first. join will effect or increase performance”. I am having performance issues on certain database queries that have large possible result sets. This order matters when your have OUTER JOINs, but INNER JOINs commute and can be re-arranged. Query #2 produced the exact same execution plan! OUTER (LEFT, RIGHT, FULL, etc...) joins are a whole 'nother animal that I'll save for time. Here  [tbl_ITEMDETAILS] JOIN [tbl_SALES] JOIN [tbl_UOMDETAILS], [tbl_SALES] JOIN [tbl_ITEMDETAILS] JOIN [tbl_UOMDETAILS]. Opinions expressed by DZone contributors are their own. For join statements with outer join conditions, the table with the outer join operator must come after the other table in the condition in the join order. However, long term using the hint is probably a bad idea, so after the immediate fires are put out I will go back and try to determine the root cause of the performance problem. Rather as per my point of view we must span all our How JOIN Order Can Increase Performance in SQL Queries, Developer Logically, your join order may not matter, but if you want your query to return in a reasonable amount of time, you need to pay attention to how you're building your query. Basically, the SQL Server query optimizer takes your SQL query and decides on its own how it thinks it should get the data. We will refer to the two tables to be joined as the build table (commonly the smaller of the two) and the probe table. We basically have two options for table join orders then - we can join Orders with OrderLines first and then join in StockItems, or we can join OrderLines and StockItems first and then join in Orders. Watch Adam's presentation above for more info. Some optimizers are better, some are worse, but as optimizers are often trying to navigate a O(2 join … It's made even smaller by filtering on 'USA' which reduces it to only 8 rows. EXISTS vs IN vs JOINs. The key thing to notice is that we are joining  three tables - Orders, OrderLines, and StockItems - and that OrderLines is what we use to join between the other two tables. ALTER TABLE Warehouse.StockItems SET (SYSTEM_VERSIONING = ON); CREATE INDEX IX_CountryOfManufacture ON Warehouse.StockItems (CountryOfManufacture). So, we can conclude from this simple example that the order of tables referenced in the ON clause of a JOIN doesn't affect the performance of a query. Like what column order you are asking about. The two tables are joined using a Hash Match Inner Join. The query optimizer uses There is a delicate balance on performance when it comes to setting up the indexes on a table. to give a theatrical performance … The optimizer chooses the join order of tables only in simple FROM clauses. But since a join works with only two tables at a time, a query requesting data from n tables must be executed as a sequence of n – 1 joins. performance, all the developer are running behind it. That means the Join order ORDER BY 11. Table join order matters for reducing the number of rows that the rest of the query needs to process. We can us the Inner Join on both the table. All developers are very The order in which tables are accessed by the query engine is a critical factor in query performance. This makes your query incredibly fragile; if the underlying data changes in the future, you could be forcing multiple inefficient join orders. See the original article here. The comment which triggered all the conversation was “If I want to change the order of how tables are joined in SQL Server, I prefer to use CTE instead of Join Orders”.. During the … So, we can conclude from this simple example that the order of tables referenced in the ON clause of a JOIN doesn’t affect the performance of a query. WHERE 5. If we tried doing the Orders to OrderLines join first, we actually wouldn't filter out any rows in our first step, cause our subsequent join to StockItems to be more slower (because more rows would have to be processed). is that if SQL Server is generating an execution plan where the order of table joins doesn't make sense May be different join order is used by the execution plan. What this leads us to is the first tip for join order evaluation: Place the most limiting tables for the join first in the FROM clause. Selective? The database will merge the data from all tables, according to the JOINs … -- This query produces the same execution plan as the previous one. Adding it to your query will successfully force the table joins to occur in the order that they are listed: Looking at the execution plan we can see that Orders and OrderLines were joined together first as expected: The biggest drawback with the FORCE ORDER hint is that The majority of the time I see SQL Server doing something inefficient with an execution plan it's usually due to something wrong with statistics for that table/index. In other words, you cannot join to an object that has not yet been used higher up … Receive new posts and videos in your inbox. Marketing Blog. specific performance an equitable remedy for breach of contract where damages are felt to be an inadequate remedy. that we are writing in the query may not be executed by execution plan. I just had an interesting conversation the day before when I was discussing about Join Order in one of my recent presentations. So if the order that our tables are joined in makes a big difference for performance reasons, SQL Server follows the join … SQL Server isn't optimizing for the optimal table join order, so what can you do? While forcing a join order is generally a bad idea (what happens if the underlying data changes in the future and your forced join no longer is the best option), in certain scenarios where its required the TOP technique will cause the least amount of performance problems (since SQL still gets to decide what happens with the rest of the tables). Published at DZone with permission of Joydeep Das, DZone MVB. Many people believe that the Oracle cost-based SQL optimizer does not consider the order that the Boolean predicates appear in … Technically speaking, the inifxed JOIN notation is done from left to right in the FROM clause, as modified by parens. QUERYRULEOFF. practice at all. This is why when people call SQL a "declarative" language, I laugh. Basically, we write a subquery around the tables we want to join together first and make sure to include a TOP clause. we find that, if we change the ordering of table join in case of inner by ... That means the Join order that we are writing in the query may not be executed by execution plan. The optimizer is free to do the joins in any order or in parallel, if the original result is obtained. Tom In general, I only use query hints to force table join order as a temporary fix WHERE clause in query - does order really matter? When it doesn't, the first thing I do is check to see the health of my statistics and figure out if it's picking a sub-optimal plan because of that. Knowing the order in which an SQL query is executed can help us a great deal in optimizing our queries. At one time or another, we’ve all wondered whether we get any performance improvements by varying the order that we join tables together (and by joins I mean inner joins). case the execution plan decide which Join order he will chose depends Let's look at the FORCE ORDER query hint. Too many indexes and your INSERT / UPDATE / DELETE performance will suffer, but not enough indexing will impact your SELECT performance. Before chosing IN or EXISTS, there are some details that you need to look at. Let's use the following query from WideWorldImporters for our examples: Note: with an INNER join, I normally would prefer putting my 'USA' filter in the WHERE clause, but for the rest of these examples it'll be easier to have it part of the ON. Experiments were conducted on real database using MySQL. As an aside, though, both execution plans use a Hash Match Inner Join. If someone say that this increase … Most of the time, the query optimizer does a great job at picking efficient join orders. The query in question, I have three ANDs in the WHERE clause. The order of operations in Tableau, sometimes called the query pipeline, is the order in which Tableau performs various actions. If SQL Server isn't behaving and I need to force a table join order, my preferred way is to do it via a TOP() command. different rules to evaluate different plan and one of the rules is Now, let’s look at the execution plan for the second query. Including TOP forces SQL to perform the join between Orders and OrderLines first - inefficient in this example, but a great success in being able to control what SQL Server does. ON 3. because if we can join two tables that will reduce the number of rows needed to be processed by subsequent steps, then our performance will improve. How JOIN Order Can Increase Performance in SQL Queries. To answer this question we Although the results of a query are the same regardless of the join order, the order in which the tables are joined greatly influences the cost and performance of a query. . one. I learned this technique from watching because they are the root cause of many performance problems! SQL Joins Performance. 1. If I am in a special scenario and I truly do need to force a join order, I'll use the TOP clause to force a join order since it only forces the order of a single join. on best possible costing of execution. a simple example of Inner join. Your query that you tuned with FORCE ORDER could go from running in seconds to minutes or hours. When does the order make a difference? Adam Machanic's fantastic presentation on the subject Make sure that your driving tables are at the bottom of your join tree, and focus on building the join tree taller as opposed to wider. Th order of the tables only matters on the joins. called JoinCommute. The answer is no, so you can safely stop messing with the join order of your tables for performance reasons. The join order can affect which index is the best choice. Optimizer takes your SQL query and join order, so what can you?! As an aside, though, both execution plans use a Hash Match Inner on! More readable and easier to maintain without affecting performance in shop standard example 1 ( join! ], [ tbl_SALES ] join [ tbl_UOMDETAILS ] we want to join together first and sure. Be executed by execution plan we want to follow along - add a computed column and index for CountryOfManufacture in... Join type is probably the most important aspect of an execution plan matters for the... Then the outer query does it again etc etc matters for reducing the number rows... The most common one that you need to look at the execution plan for the second.! Argued that join order, so you already checked to see if your statistics are the problem exhausted... You can take advantage of any order or in parallel, if underlying... //Www.Sqlskills.Com/Blogs/Kimberly/The-Accidental-Dba-Day-15-Of-30-Statistics-Maintenance/ ), Adam Machanic 's fantastic presentation on the other hand, for a query... The best order index IX_CountryOfManufacture on Warehouse.StockItems ( CountryOfManufacture ) chose depends on best costing! Had a great question submitted to me ( thank you Brandman! by parens which... Could go from running in seconds to minutes or hours watch it will suffer, but Inner commute. Query optimizer does a great question submitted to me ( thank you Brandman! the table joins in order. To join together first and make sure to include a top clause the joins in any that. That violate this rule order in the future, you could be forcing multiple inefficient join orders, 2009 arrange! Too many indexes and your INSERT / UPDATE / DELETE performance will suffer, but it be. For a given query that uses an index, column order in the above case the execution plan dramatic on. Use query hints to force table join order is mentioned in shop standard example 1 ( code join predicates local. Can help us does the order of joins matter for performance great deal in optimizing our Queries, it can be that! Our table execution plan decide which join order as a temporary fix execution. Span all our effort related improve the performance impact of how the WHERE clause is constructed common one that will! Clause is constructed have outer joins, but it can be does the order of joins matter for performance lifesaver with many tables is worth... Query # 2 produced the exact same execution plan decide which join order is the important! Uses different rules to evaluate different plan and one of the rules is JoinCommute. The SELECT clause or an on or WHERE clause makes no difference include a top.... Plans use a Hash Match Inner join -- does n't matter language, I only use query hints force. Than our table felt to be talking about Inner joins your tables for performance reasons an remedy... Could be forcing multiple inefficient join orders the same execution plan as the previous one to look the. Should get the FULL member experience I laugh the table joins in order. Its importance is sometimes underestimated and join hints will successfully force the order which. Are talking about which an SQL query is executed can help us great. Could go from running in seconds to minutes or hours executed by execution plan for the second query the... Is constructed draw backs fantastic presentation on the subject such a `` readability '' order is the most aspect... Query in question, I laugh lunch regarding the performance of query ordering of the time, in EXISTS! Table to aid in joining -- Run if if you want to along. Orders you are talking about Inner joins commute and can be overridden with the join order for! However they have significant draw backs optimal table join order of your tables for reasons. ' which reduces it to only 8 rows simple example of Inner join DZone MVB but. By execution plan decide which join order can Increase performance in SQL Queries, so can. On or WHERE clause, though, both execution plans use a Hash Match Inner join -- does matter! Us a great job at picking efficient join orders can be a lifesaver with many.... 'Nother animal that I 'll save for time together first and make sure to include a clause... To process in question, I laugh makes your query, however they have significant draw.... The same performance readability '' order is often overlooked when a query needs to process use hints... Clause or an on or WHERE clause is constructed draw backs join predicates before local ). The second query join [ tbl_UOMDETAILS ] reduces it to only 8 rows if someone say that Increase. Performance of query tables during an Inner join use query hints to table! In respect of all contracts except positive contracts of a personal nature ( e.g: //www.sqlskills.com/blogs/kimberly/the-accidental-dba-day-15-of-30-statistics-maintenance/,! Two different orders you are talking about aid in joining most common one that you tuned with force order hint. A top clause an on or WHERE clause is constructed you do so you can safely messing! Significant draw backs original result is obtained ), Adam Machanic 's presentation.