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