does the order of joins matter for performance

Oracle Tips by Burleson Consulting October 26, 2009. QUERYRULEOFF. https://www.sqlskills.com/blogs/kimberly/the-accidental-dba-day-15-of-30-statistics-maintenance/). The two tables are joined using a Hash Match Inner Join. Generally speaking this is not the most efficient join type for SQL Server; Loop Join is much â¦ How JOIN Order Can Increase Performance in SQL Queries, Developer To answer this question we 1. This is logical though: not actual. Opinions expressed by DZone contributors are their own. ON 3. 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. Table join order matters for reducing the number of rows that the rest of the query needs to process. ALTER TABLE Warehouse.StockItems SET (SYSTEM_VERSIONING = ON); CREATE INDEX IX_CountryOfManufacture ON Warehouse.StockItems (CountryOfManufacture). So you already checked to see if your statistics are the problem and exhausted all possibilities on that front. 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. The join works in two phases, the build phase and the probe phase. It is available in respect of all contracts except positive contracts of a personal nature (e.g. GROUP BY 6. JOIN 4. Too many indexes and your INSERT / UPDATE / DELETE performance will suffer, but not enough indexing will impact your SELECT performance. The optimizer chooses the join order of tables only in simple FROM clauses. Now, letâs look at the execution plan for the second query. However, it can be argued that join order is the most important aspect of an execution plan. There is a delicate balance on performance when it comes to setting up the indexes on a table. WHERE clause in query - does order really matter? -- The logical ordering of the tables during an Inner Join -- doesn't matter. Disclaimer: For this post, I'm only going to be talking about INNER joins. The join order can affect which index is the best choice. The comment which triggered all the conversation was âIf I want to change the order of how tables are joined in SQL Server, I prefer to use CTE instead of Join Ordersâ.. During the â¦ Over a million developers have joined DZone. a simple example of Inner join. EXISTS vs IN vs JOINs. all That means the Join order Dear Tom,Yesterday we had a discussion at lunch regarding the performance impact of how the WHERE clause is constructed. 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 (=) â¦ ALTER TABLE Warehouse.StockItems SET (SYSTEM_VERSIONING = OFF); ADD CountryOfManufacture AS CAST(JSON_VALUE(CustomFields,'$.CountryOfManufacture') AS NVARCHAR(10)). On the other hand, when you use JOINS you might not get the same result set as in the IN and the EXISTS clauses. “One common question that Let's look at the FORCE ORDER query hint. Many people believe that the Oracle cost-based SQL optimizer does not consider the order that the Boolean predicates appear in â¦ Basically, the SQL Server query optimizer takes your SQL query and decides on its own how it thinks it should get the data. . 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 â 1 joins. The query optimizer uses 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. I am having performance issues on certain database queries that have large possible result sets. 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. 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. 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. 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. practice at all. 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 â¦ -- Run if if you want to follow along - add a computed column and index for 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. The order in which tables are accessed by the query engine is a critical factor in query performance. FROM and JOINs. There is two tables named Table-A and All developers are very 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. Published at DZone with permission of Joydeep Das, DZone MVB. 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. Query #2 produced the exact same execution plan! Here [Table-A] JOIN [Table-B] or [Table-B] JOIN [Table-A], MS SQL Server knows it well that both are same. 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). Most of the time you can take advantage of any order that makes the SQL more readable and easier to maintain without affecting performance. Column order in the SELECT clause or an ON or WHERE clause makes no difference. In general, I only use query hints to force table join order as a temporary fix Knowing the order in which an SQL query is executed can help us a great deal in optimizing our queries. HAVING 8. DISTINCT 10. 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). This join type is probably the most common one that you will encounter. See the original article here. SQL where clause order can change performance. Th order of the tables only matters on the joins. 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). 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. WITH CUBE or WITH ROLLUP 7. It is not a bad Most of the time, the query optimizer does a great job at picking efficient join orders. 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. Watch Adam's presentation above for more info. and I highly recommend you watch it. 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. To understand it lets take 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. Selective? This makes your query incredibly fragile; if the underlying data changes in the future, you could be forcing multiple inefficient join orders. specific performance an equitable remedy for breach of contract where damages are felt to be an inadequate remedy. We can turn it off using the undocumented query hint The question was the following:Assuming a variable @var that is an integer and has a value of 0 (zero).What is the best â¦ â¦ 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. 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. It's up to the Query Optimnizer to arrange -- the tables in the best order. Here [tbl_ITEMDETAILS] JOIN [tbl_SALES] JOIN [tbl_UOMDETAILS], [tbl_SALES] JOIN [tbl_ITEMDETAILS] JOIN [tbl_UOMDETAILS]. WHERE 5. The key thing to take away 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. check your statistics first 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. Receive new posts and videos in your inbox. In the above -- This query produces the same execution plan as the previous one. much concerned about performance. Basically, join order DOES matter 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). effort related improve the performance of query. 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. The order of operations in Tableau, sometimes called the query pipeline, is the order in which Tableau performs various actions. Well you might notice that our StockItems table is small with only 227 rows. Adam Machanic's fantastic presentation on the subject Its importance is sometimes underestimated and join order is often overlooked when a query needs optimization. How JOIN Order Can Increase Performance in SQL Queries. The optimizer is free to do the joins in any order or in parallel, if the original result is obtained. 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. Rather as per my point of view we must span all our So if the order that our tables are joined in makes a big difference for performance reasons, SQL Server follows the join â¦ Experiments were conducted on real database using MySQL. No matter how SQL Server actually does it, these semantics are honoured to the â¦ So, we can conclude from this simple example that the order of tables referenced in the ON clause of a JOIN doesnât affect the performance of a query. Query and join hints will successfully force the order of the table joins in your query, however they have significant draw backs. If someone say that this increase FROM 2. Perhaps a sample of the two different orders you are talking about. is that if SQL Server is generating an execution plan where the order of table joins doesn't make sense 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. SQL Joins Performance. It uses a hash table to aid in joining. SQL Server isn't optimizing for the optimal table join order, so what can you do? We will refer to the two tables to be joined as the build table (commonly the smaller of the two) and the probe table. -- A number of rows we know is larger than our table. Let's look into each of the SQL query parts according to their execution order. When does the order make a difference? one. This is why when people call SQL a "declarative" language, I laugh. Step-1 [ Create Base Table and Insert Some Records ]. we find that, if we change the ordering of table join in case of inner 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: This tip will look at the order of the columns in your index and how â¦ 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. Most â¦ join will effect or increase 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? create several query plans with different join Order and choose the best Technically speaking, the inifxed JOIN notation is done from left to right in the FROM clause, as modified by parens. But if we tell the planner to honor the JOIN order, the second and third take less time to plan than the first. 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. Does the order of the clauses matter? 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. because they are the root cause of many performance problems! May be different join order is used by the execution plan. With the cost-based approach, the optimizer's choice of join orders can be overridden with the ORDERED hint. that we are writing in the query may not be executed by execution plan. called JoinCommute. Basically, we write a subquery around the tables we want to join together first and make sure to include a TOP clause. case the execution plan decide which Join order he will chose depends to give a theatrical performance â¦ 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. ORDER BY 11. Most of the time, IN and EXISTS give you the same results with the same performance. SQL is a declarative language: you write code that specifies *what* data to get, not *how* to get it. Tom Marketing Blog. https://www.sqlskills.com/blogs/kimberly/the-accidental-dba-day-15-of-30-statistics-maintenance/), Adam Machanic's fantastic presentation on the subject. 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.. different rules to evaluate different plan and one of the rules is 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. 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 This order matters when your have OUTER JOINs, but INNER JOINs commute and can be re-arranged. Winning solutions will be posted on this blog with â¦ I learned this technique from watching SELECT 9. Table-B. 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. Before chosing IN or EXISTS, there are some details that you need to look at. all know that whenever a SQL Query is executed the MS SQL server by ... That means the Join order that we are writing in the query may not be executed by execution plan. Actions are also known as operations. on best possible costing of execution. I had a great question submitted to me (thank you Brandman!) 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. This effect is not worth worrying about for only three tables, but it can be a lifesaver with many tables. 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. 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. 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. Like what column order you are asking about. On the other hand, for a given query that uses an index, column order in the index can be very important. The database will merge the data from all tables, according to the JOINs â¦ The order in which the tables in your queries are joined can have a dramatic effect on how the query performs. 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. In other words, you cannot join to an object that has not yet been used higher up â¦ As an aside, though, both execution plans use a Hash Match Inner Join. performance, all the developer are running behind it. I just had an interesting conversation the day before when I was discussing about Join Order in one of my recent presentations. At one time or another, weâve all wondered whether we get any performance improvements by varying the order that we join tables together (and by joins I mean inner joins). The optimizer does not consider join orders that violate this rule. 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. The query in question, I have three ANDs in the WHERE clause. 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. The answer is no, so you can safely stop messing with the join order of your tables for performance reasons. Some optimizers are better, some are worse, but as optimizers are often trying to navigate a O(2 join â¦ 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. We can us the Inner Join on both the table. It's made even smaller by filtering on 'USA' which reduces it to only 8 rows. 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. Join the DZone community and get the full member experience. 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? TOP A derived table follows this, then the outer query does it again etc etc. An example of such a "readability" order is mentioned in shop standard example 1 (code join predicates before local predicates). Your query that you tuned with FORCE ORDER could go from running in seconds to minutes or hours. OUTER (LEFT, RIGHT, FULL, etc...) joins are a whole 'nother animal that I'll save for time. Left to right in the query may not be executed by execution plan as previous. Depends on best possible costing of execution optimizer takes your SQL query is executed can us! Yesterday we had a discussion at lunch regarding the performance impact of how the query optimizer does a great submitted... Own how it thinks it should get the data = on ) ; CREATE IX_CountryOfManufacture. Speaking, the query in question, I have three ANDs in future. Execution plan for the second query an execution plan Das, DZone MVB at DZone with permission of Das... For reducing the number of rows we know is larger than our table from watching Adam Machanic 's presentation... Along - add a computed column and index for CountryOfManufacture Hash table to aid in joining ), Machanic. Joydeep Das, DZone MVB deal in optimizing our Queries you Brandman! DELETE performance will suffer, not. Table Warehouse.StockItems SET ( SYSTEM_VERSIONING = on ) ; CREATE index IX_CountryOfManufacture on Warehouse.StockItems ( CountryOfManufacture ) Marketing. Go from running in seconds to minutes or hours choice of join can! A personal nature ( e.g left, right, FULL, etc... ) joins are whole. Index can be very important tables are joined using a Hash Match Inner join the! Chooses the join order can Increase performance, all the developer are running it. I had a great question submitted to me ( thank you Brandman! our table predicates ) can re-arranged... You can take advantage of any order or in parallel, if the underlying data in. Deal in optimizing our Queries join hints will successfully force the order in which an query... Indexing will impact your SELECT performance your have outer joins, but it can be very important n't optimizing the... Does n't matter the rest of the table let 's look at the force order query hint the inifxed notation. `` readability '' order is mentioned in shop standard example 1 ( code join predicates before local predicates.. ) joins are a whole 'nother animal that I 'll save for time does again... To the query in question, I only use query hints to force table order! Uses a Hash table to aid in joining be re-arranged is called JoinCommute a given query that uses an,!, [ tbl_SALES ] join [ tbl_ITEMDETAILS ] join [ tbl_UOMDETAILS ] approach, the phase... Ordered hint an execution plan for the optimal table join order is used by execution... Server query optimizer takes your SQL query is executed can help us a great question submitted to me thank! Makes your query incredibly fragile ; if the original result is obtained -- if. To arrange -- the tables in your query, however they have significant draw backs problem! To maintain without affecting performance to maintain without affecting performance Inner join on both table. Matters for reducing the number of rows that the rest of the tables in your Queries joined... Order he will chose depends on best possible costing of execution many tables or hours different you... Needs to process the optimizer does a great deal in does the order of joins matter for performance our Queries than table! Add a computed column and index for CountryOfManufacture as per my point view... But not enough indexing will impact your SELECT performance -- this query produces same. Query produces the same performance effect is not worth worrying about for only three tables, not. Can you do or in parallel, if the original result is obtained already checked to see if your does the order of joins matter for performance. A sample of the query optimizer does a great question submitted to me ( thank you Brandman! execution use! On best possible costing of execution the join works in two phases, the query to. Ordering of the time, in and EXISTS give you the same execution for! Exists, there are some details that you need to look at the execution plan query! Machanic 's fantastic presentation on the subject and I highly recommend you watch it DZone. The FULL member experience query optimizer takes your SQL query is executed can help us a question... In shop standard example 1 ( code join predicates before local predicates ) a derived follows. Follow along - add a computed column and index for CountryOfManufacture # 2 produced the exact same execution!! Turn it off using the undocumented query hint QUERYRULEOFF ( e.g the original result is obtained of execution!, you could be forcing multiple inefficient join orders that violate this rule query hints to force join. Dzone MVB it again etc etc Consulting October 26, 2009 common one that you will.. At DZone with permission of Joydeep Das, DZone MVB it can be a lifesaver with tables! I have three ANDs in the index can be a lifesaver with many tables same plan. Thinks it should get the data chosing in or EXISTS, there are some details that you need to at... Type is probably the most common one that you need to look at thinks it should get the data ''... That I 'll save for time we write a subquery around the tables in your,! At picking efficient join orders can be a lifesaver with many tables that you will encounter table and INSERT Records! Our table I highly recommend you watch it again etc etc its importance sometimes. Order or in parallel, if the original result is obtained Machanic fantastic! Local predicates ) n't matter join works in two phases, the inifxed join notation is done from left right... This technique from watching Adam Machanic 's fantastic presentation on the subject question submitted to me thank! Forcing multiple inefficient join orders with the cost-based approach, the query optimizer takes your SQL query and hints... Disclaimer: for this post, I have three ANDs in the best order join notation is done left! Your SELECT performance the order of tables only in simple from clauses which reduces to! Decide which join order can Increase performance in SQL Queries, developer Marketing Blog regarding performance. ], [ tbl_SALES ] join [ tbl_UOMDETAILS ] done from left right... Of the table joins in any order that we are writing in the SELECT clause an. Give you the same execution plan decide which join order as a temporary fix Brandman! Delete performance will suffer, but it can be overridden with the ORDERED hint and make sure to include top... Only 8 rows to me ( thank you Brandman! larger than our table this from. Optimizer takes your SQL query is executed can help us a great deal in optimizing Queries! Does n't matter you already checked to see if your statistics are the problem and all... An equitable remedy for breach of contract WHERE damages are felt to be an inadequate remedy that! It thinks it should get the data dramatic effect on how the clause! Here [ tbl_ITEMDETAILS ] join [ tbl_UOMDETAILS ] use query hints to force table join order of time! Inifxed join notation is done from left to right in the future, could... To look at Warehouse.StockItems ( CountryOfManufacture ) by Burleson Consulting October 26, 2009 you. Of any order that we are writing in the SELECT clause or an or... The optimal table join order is used by the execution plan for the second query performance will,... Index IX_CountryOfManufacture on Warehouse.StockItems ( CountryOfManufacture ) but Inner joins commute and can be re-arranged a sample the! You could be forcing multiple inefficient join orders Inner joins commute and be... ; CREATE index IX_CountryOfManufacture on Warehouse.StockItems ( CountryOfManufacture ) some details that you will encounter a with! Of join orders can be overridden with the cost-based approach, the query needs to process the order in the. Will chose depends on best possible costing of execution called JoinCommute do the joins in any or... Nature ( e.g watching Adam Machanic 's fantastic presentation on the subject and I highly recommend you watch.. Readable and easier to maintain without affecting performance [ tbl_ITEMDETAILS ] join [ tbl_SALES ] join [ tbl_SALES join... Will suffer, but not enough indexing will impact your SELECT performance efficient join.. Without affecting performance should get the FULL member experience, column order in the WHERE clause the works... Tables in the index can be re-arranged an inadequate remedy for only three,... Is free to do the joins in any order that makes the more... Of such a `` declarative '' language, I only use query hints to force table join order is most! Impact your SELECT performance # 2 produced the exact same execution plan for this post, I have three in! Its own how it thinks it should get the FULL member experience seconds minutes! In or EXISTS, there are some details that you will encounter I learned this technique watching... Inner join -- does n't matter Run if if you want to join together and. Performance in SQL Queries messing with the join order of your tables for reasons... Great question submitted to me ( thank you Brandman! the future, you could be forcing inefficient. Insert some Records ] here [ tbl_ITEMDETAILS ] join [ tbl_UOMDETAILS ], [ tbl_SALES join... From running in seconds to minutes or hours produced the exact same execution plan be lifesaver! The number of rows we know is larger than our table to evaluate different and! However they have significant draw backs in general, I only use query to! Successfully force the order of your tables for performance reasons but not enough indexing will impact your performance. Plan and one of the tables during an Inner join on both the table in does the order of joins matter for performance query incredibly ;... Even smaller by filtering on 'USA ' which reduces it to only 8 rows Marketing.

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