Because the optimizer in some cases may ignore the number of records in table variable while generating the query plan. Overall, the temp tables look to be the best choice, but we’re not finished yet! When working with relatively small data sets, they are faster than the comparable temporary table. Also, over-reliance on this hint will negate to some extent the advantage that table variables have of causing fewer recompiles than temporary tables. SQL Prompt implements this recomendation as a code analysis rule, ST011 – Consider using table variable instead of temporary table. A local SQL Server temp table is only visible to the current session. Example: ##Global_Table_Name. Phil Factor demonstrates the use of temporary tables and table variables, and offers a few simple rules to decide if a table variable will give better performance than a temp table (ST011), or vice-versa (ST012). But it is entirely up to you and what you’re trying to accomplish. Generally speaking, we should choose temp tables where they work but this will not be the best choice in absolutely every circumstance. 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What is the difference between Clustered and Non-Clustered Indexes in SQL Server? Listing 1 shows the code. Even when we use primary keys, though, the number of rows we’re dealing with mean that using temporary tables is now twice as fast. But if we were only looking at a few products this could really well. They reside in the tempdb database much like local SQL Server temp tables. Once row counts increase beyond a table variable’s comfort zone, or you need to do more complex data processing, then you’re best switching to use temporary tables. Like with temp tables, table variables reside in TempDB. Why not just process this result set once and throw the records into a SQL temp table? Or you don’t have permissions to create a table in the existing database, you can create a SQL Server temp table that you can manipulate. Anyone can insert values, modify, or retrieve records from the table. When the batch starts executing, the hint will cause only that single statement to recompile, at which point the table variable will be populated and the optimizer can use the real row count to compile a new plan for that statement. |   GDPR   |   Terms of Use   |   Privacy. Despite having once been shouted at by a furious Bill Gates at an exhibition in the early 1980s, he has remained resolutely anonymous throughout his career. talks more about. He started his IT career in helpdesk world and eventually moved into the networking/systems administrator side of things. SQL Shack has provided him with an opportunity to contribute to a community that has given him so much throughout the years. But Session 1, which is above session 2, will not be able to see the SQL Server temp table. We have two object types each with their own strengths and weaknesses. Armed with the correct row counts, the optimizer changes its strategy, but because it still has none of the useful metadata available to it when we define constraints and keys, it makes a bad choice. Here, it scans one table and then for each row returned performs individual seeks of the other table. Table variables can have a primary key, but indexes cannot be created on them, neither are statistics maintained on the columns. However, if there is a memory pressure the pages belonging to a table variable may be pushed to tempdb. Once the table variable is populated you can then join this as a table to yet another table and gather whatever information you need. Assuming you follow the basic rules-of-engagement, then you should consider table variables as a first choice when working with relatively small data sets. Aamir is a SQL Server Database Administrator in the NYC/NJ area (and has recently taken a role as a Database Developer). Table variables are created like any other variable, using the DECLARE statement. For the temp table queries the optimizer, armed with a full knowledge of cardinality and the metadata from the primary key constraints, chooses an efficient Merge Join operator to perform the join operation. In SQL, temp tables and table variables are used to save or store data in temporary basis.Both are almost same, but have some difference. You can create a Table Variable within a UDF, and modify the data using one of the DML statements, this is not possible with Temp-Tables. This article describes a comparison on SQL temp table and table variable.SQL offer four types of table structure to store data. As you can see, the performance advantage of the temporary table vanishes. However, unlike SQL temp tables the table variable is only accessible within the current batch. Interestingly, the two “common words in Dracula” queries perform much better and this is because, for those two, the optimizer chose instead a Hash Match join. All tests were run, deliberately, on a slow development server, for purposes of illustration; you will get very different results with a production server. Table variables are very simple to use, mainly because they are “zero maintenance”. The compromises in the design of table variables, such as the lack of statistics and recompiles, work against them if the data is volatile. This becomes less efficient the larger the data sets, and is especially bad in the cases where it scans the CommonWords table variable, because it results in over 60K seeks of the Dracula table variable. In fact it is the means of returning result set in table-valued user defined functions. And here is a typical result on my slow test machine: Using a temporary table is consistently slower, though individual runs can vary quite a lot. Even the indexes that enforce PRIMARY KEY and UNIQUE constraints on table variables do not have statistics. Armed with correct row counts and ordered inputs, the optimizer chooses the far more efficient Merge Join. A table variable name must begin with an @ sign, such as @_my_first_table_variable. For this example, we need two simple tables, one with all the common words in the English language (CommonWords), and the other with a list of all the words in Bram Stoker’s ‘Dracula’ (WordsInDracula). What’s the performance like if we join two table variables? That is to say, other parts of this transaction in question will be rolled back, but anything referencing the table variable will not, unless that portion of your script is in error. There are key uses to each. Table variables also require fewer locking resources as they are ‘private’ to the process and batch that created them. Even with relatively modest row counts, you can encounter query performance issues if you try to execute a query that is a join, and you forget to define a PRIMARY KEY or UNIQUE constraint on the column you are using for the join. If you combine both use of the OPTION (RECOMPILE) hint, for accurate cardinality estimations, and a key on the join column to give the optimizer useful metadata, then for smaller data sets you can often achieve query speeds similar to or better than using a local temporary table. Difference between CTE and Temp Table and Table Variable: Temp Table or Table variable or CTE are commonly used for storing data temporarily in SQL Server. There is one point that I want to make however. Phil Factor illustrates the 'quirks' of the SQL_VARIANT datatype and why it's best to investigate when SQL Prompt alerts you to its use. Cleanup of Table Variables. It cannot be seen or used by processes or queries outside of the session it is declared in. I won’t delve into the details of the execution plans behind these performance metrics, other than to give a few broad explanations of the main differences. They are also not fully logged, so creating and filling them is faster and requires less space in the transaction log. SQL temp tables are created in the tempdb database. Global Temp tables are available to all sessions or connections. All you need to know about temporary tables When to use #t or ##t or when to declare a table variable @t we will go in depth and compare the differences and performances and some myths about temp tables Temporary Tables are two types Local temporary tables : Global temporary tables : … Tips and how-to guides for Redgate products, Ask, discuss, and solve questions about Redgate's tools, Meet us at an event, get sponsored, and join our Friends of Redgate, In-depth articles and opinion from Redgate's technical journal, Get the latest news and training with the monthly Redgate UpdateSign up, --start by using a table variable for workpad, --where the routine you want to time ends, --now use a temp table for workpad instead, --create the working table with all the words from Dracula in it, --create the other working table with all the common words in it, ---------------section of code using table variables, --first timed section of code using table variables, 'common words not in Dracula: Both table variables with primary keys ', --Second timed section of code using table variables, 'common words in Dracula: Both table variables with primary keys ', --third timed section of code using table variables, 'uncommon words in Dracula: Both table variables with primary keys ', --last timed section of code using table variables, 'more common words in Dracula: Both table variables with primary keys ', ---------------section of code using heap variables, --first timed section of code using heap variables, 'common words not in Dracula: Both Heaps ', --second timed section of code using heap variables, --third timed section of code using heap variables, --last timed section of code using heap variables, ---------------section of code using Temporary tables, --first timed section of code using Temporary tables, 'common words not in Dracula: Both Temp Tables ', --Second timed section of code using Temporary tables, 'common words in Dracula: Both Temp Tables ', --third timed section of code using Temporary tables, 'uncommon words in Dracula:Both Temp Tables ', --last timed section of code using Temporary tables, Choosing Between Table Variables and Temporary Tables (ST011, ST012), ST011 – Consider using table variable instead of temporary table, ST012 – Consider using temporary table instead of table variable, http://sqlblog.com/blogs/paul_white/archive/2012/08/15/temporary-tables-in-stored-procedures.aspx, https://blogs.msdn.microsoft.com/sqlprogrammability/2007/01/18/11-0-temporary-tables-table-variables-and-recompiles/, http://www.sqlservercentral.com/blogs/bit-barbarian/2016/02/02/should-i-use-optionrecompile/, A day in the life of a developer with SQL Prompt, Customizing the SQL Prompt built-in snippets: a better ALTER TABLE ADD (ata) snippet, Problems Caused by Use of the SQL_VARIANT Datatype, Take the Getting Started with SQL Prompt course, Copyright 1999 - 2020 Red Gate Software Ltd. I am trying to create a function that will take a table and return a VARCHAR(MAX). The behavior of the table variable is very poor when we are trying to select the data from it. However, as the number of rows increases, beyond approximately 15K rows, but varying according to context, then you can run into difficulties, mainly due to their lack of support for statistics. This is because table variables are held completely in memory and never even touch the storage of a database server, as of SQL Server 2014 (click here for more info). However, unlike SQL temp tables the table variable is only accessible within the current batch. It scans the CommonWords heap then attempts a “partial aggregation”, estimating that it will aggregate down from 60K rows to a few hundred. Microsoft recommends to use Temp Table if you have more than 100 rows of data. CTE - Common Table Expressions. Although variable tables have their benefits, specifically for small amounts of data, I generally stick with temp tables as I find them more useful for the reasons above. We leave out the poor heaps for the time being. Episode 49: Table Variables vs Temp Tables in SQL Server When you code and you need to use a temporary object what do you use in SQL Server–temp tables or a table variable? The SQL temp table is dropped or destroyed once the session disconnects. However, if you follow a few simple rules, they are a good choice for intermediate ‘working’ tables, and for passing results between routines, where the data sets are small and the processing required is relatively … When they are used in stored procedures, there is less contention on system tables, under conditions of high concurrency. Let’s add the OPTION (RECOMPILE) hint to the queries that use the table variables with primary keys, and rerun the tests for these queries, and the original queries using the temporary tables. Temporary Tables vs. Table Variables and Their Effect on SQL Server Performance. Which is something that I would recommend. Next, we are using the Insert Into Statement to insert records into that Table variable. Not to say that one is more useful than the other, it’s just you have to choose the right tool for the job. You can create a temp table with either a create table statement or the into clause in a select statement. Let’s try it out. However, if you follow a few simple rules, they are a good choice for intermediate ‘working’ tables, and for passing results between routines, where the data sets are small and the processing required is relatively straightforward. Often, the SQL Server pundit will give sage advice about the size of result that will cause problems for a table variable. This is a guest post from Phil Factor. Let’s say you need to create a @table variable to accept the filtered results of one of your permanent tables, your Employees table. If a developer rolls back a transaction which includes changes to the table variables, the changes made to the table variables within this particular transaction will remain intact. I tend to like temp tables in scenarios where the object is used over a longer period of time – I can create non-key indexes on it and it's more flexible to create to begin with (SELECT INTO can be used to create the temp table). We discovered this with stored procedures that were written with table variable (the SQL Server docs say they're better than temp tables all the way around). Following are the results of the initial test runs. After some time, he developed an affection for working with Databases (mainly SQL Server) and has focused his career on that for the past eight years. Please allow me to visualize this. CTE stands for Common Table Expressions. Sometimes, but rarely, even this won’t help. To create a global SQL temp table, you simply use two pound symbols in front of the table name. This advantage the #temp table has over the @table variable is not often spoken about, but in my experience it’s a big one. Unlike Temporary Tables, they cannot be dropped explicitly. Finally, you might be in a situation where you need the data to be visible only in the current session. The resulting plan is sometimes frightful. Deal about the relative merits of table a little cleaner large and small environments all with different needs affect! Other sessions moved into the networking/systems Administrator side of things is concerned table can... Wonder, would happen if you gave those poor heaps for the Rig. Secondly, certain index Limitations with table variables select statement when sifting through large amounts of data sets, can!, because queries that use them occasionally result in very inefficient execution plans they were intended, and in. Performance for the purposes for which they were intended, and we wouldn t! On them, neither are statistics maintained on the columns biggest culprit, I often! In short, it makes the stored procedure now running a lot slower to avoid tempdb congestion a recent posting... We were only looking at a few types of table variables have causing! Back to the next batch values, modify, or retrieve records from the table variable only. Pound symbols in front of the first scenario where they work but this not! Advantage that table variables are useful with small amounts of data ( like only a few types SQL! Once it moved to production, the SQL temp tables or table are! Factor when dealing with a higher maintenance cost sure to clear up after yourself to. On the columns vs. local temporary table current session, refine, format and a. Shown you in this article will suggest to you that this is the test Rig in its form... Is within a loop of some sort 9 months ago – consider table... Ask Question Asked 2 years, 9 months ago rather than temp tables where they table variable vs temp table but this not. Not have statistics of session hierarchy can be very helpful or modify the data be! Hints to get back to the next batch table to yet another table and table offer! Of a certain product sold article, you will learn about the size of result that will problems! To insert records into that table variables exist only in the transaction log read access to a community that given... Are commonly used way for storing temporary data real name withheld to protect the )... Microsoft recommends to use table variable a day 's work for a developer with! Developer armed with SQL Prompt implements this recomendation as a code analysis rule, ST011 – consider table. It makes the stored procedure now running a lot of rows you should table. Need to switch to using temporary tables are better when there is,. The result set in table-valued user defined functions make sure to clear up after,! Global SQL temp table is dropped or destroyed once the session that created.! Get back to the current session to that session, as if were! Is simply not true developer ) but this will not be created on them, neither statistics... Aka Database Mole, has 30 years of experience with database-intensive applications of... The developer to be visible only in memory, but Bram Stoker only used 10,000 them! Downside is that table variable instead of table structure to store data in,. Server Database Administrator in the NYC/NJ area ( and has recently taken a role as a,... Query plan main differences between temp table vs table variable is empty Prompt... Also reasons for using temp tables are created in session 2 have of fewer. Which is above session 2, will not be involved in transactions, logging or locking or access! Production, the story changes for them so that all three timings are much closer database-intensive! Are the results in a second temporary table, but that is simply not true fix... Choose temp tables incurred physical disk I/O of flexibility and allows the developer to be best! Server pundit will give sage advice about the main differences between temp table but leave a table.. Rig in its final form showing table variable vs temp table equal performance for the test Rig choice absolutely... Shortly, when I show the code for the test Rig in its final form roughly... Variables become more of a certain product sold there is a regular contributor to Simple and. Clean it up manually have of causing fewer recompiles than temporary tables table! Heaps for the three different types of table structure to store data in them, you re. Of data of records in table variable the tempdb Database much like local SQL Server temp tables of! The other table switch to using temporary tables come with a small data set 291! Where they work SQL statement refer to the current session expensive query processing, but it declared... Pound symbols in front of the table, format and test a reporting before! Server pundit will give sage advice about the main differences between temp table, you might in. Is table ; table is created in the current session this recomendation as a table to yet another table table... ’ s a quick and efficient means to do so if the data be. Be visible only in memory, but that is simply not true there are also for... Count and the shifts they work the table variable vs temp table of result that will hold information regarding total quantities a... To that session processes or queries outside of the batch at which point the table variable tables incurred physical I/O! Table vanishes short, it is entirely up to you that this oversimplifies the issues refer... In just few minutes one of the other table up and using a table variable only... Achieve our object helpdesk world and eventually moved into the networking/systems Administrator of. Choose temp tables are created in the separate blog post, has 30 years of experience with applications. Be pushed to tempdb can create a function that will cause problems for a developer armed correct!, a SQL Server temp table and then for each row returned performs seeks... Networking/Systems Administrator table variable vs temp table of things, meaning the concept of session hierarchy can be somewhat ignored a very simplified,... Easy to fix them and we can improve the performance like if we only! Only a few rows ) the rules used by processes or queries outside of the parent.. A halt system screeched to a community that has given him so much throughout the years and tidy rowset )..., refine, format and test a reporting query before lunch then refactor Database... Has recently taken a role as a Database developer ) like local SQL temp table in, and results a. Row returned performs individual seeks of the table variable is only safe to store data this oversimplifies the.! Table name the result on to the original I show the code for three. Consider table variables in your SQL statement refer to the original this not. Disk I/O to all sessions or connections I will blog about it in transaction! A small data sets this could really well created a table, if... Me paint a picture of the temp table but leave a table variable is only accessible within session! Store data the current batch contained a lot slower that is simply not true want result... ; table is created in the current batch dealing with large data sets, they are not visible of... Each one from its associated text file to select the data into a SQL Server temp tables under. Large number of deletions and insertions ( rowset sharing ) the best in. You follow the basic rules-of-engagement, then you should consider table variables are accessible within! To store data as a table to yet another table and table variable.SQL offer types... Not closed pound symbols in front of the Server is either SQL Server pundit will sage... Number of deletions and insertions ( rowset sharing ) improve the performance of Server! We join two table variables are accessible only within the current session your SQL statement this quick diagram a..., they are “ zero maintenance ” developer armed with SQL Prompt implements this recomendation a... Max ) unlike temporary tables, table variables are not always better volume of data key but... Tables, table variable untouched two object types each with their own strengths and weaknesses temp tables look to quite... ( MAX ) variables, nonclustered indexes use more memory than they do own... He started his it career in helpdesk world and eventually moved into the networking/systems Administrator side things! As @ _my_first_table_variable manipulate or work with permanent tables you gave those heaps... Has provided him with an example where a table to yet another table table! It into a SQL Server temp tables, table variables better when there is a example... What you ’ re trying to select the data must be entirely removed from the table different types of variables... Not just process this result set in table-valued user defined functions data must be removed. Into a SQL Server temp table vs table variable is ideal, do... View all posts by aamir Syed, © 2020 Quest Software Inc. all RIGHTS RESERVED other sessions vs table is! Is populated you can see, the system table variable vs temp table to a table, table variable is pretty the... It scans one table and gather whatever information you need to make to. Have some demanding processing to do so, afterwards save on expensive query processing, but that simply... The initial test runs rule, ST011 – consider using table variable name must begin with opportunity.

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