Is it a bad idea to use table variables? No, it is not. However, I have not seen the queries where this process took more than 100 ms. The drawback is that the execution plan is not stored and the server has to compile the query and look for an effective execution plan each time. Now, the server compiles its code before running each query and does not use the execution plan from the cache, but generates a new one, depending on the amount of data in the variable, and this usually helps a lot. With large data, the performance improvement can be significant, from tens of minutes to seconds. If we measure the query performance after that, the time will most likely be reduced for performing the search. The purpose of this option is to make SQL Server recompile the query upon each execution. This option is added once at the very end of the query following even ORDER BY. However, in my opinion, the most effective in terms of performance is to add OPTION (RECOMPILE) to the end of the query: select * There are several ways to solve this problem. I think Microsoft did not expect users to use tabular variables in this way, but there is a nice workaround. However, its execution plan cannot be cached because changing only one ID will change the whole query text as well and parameters cannot be used. There may be a thousand of such SELECT statements and the query text will be huge, but it will be executed thousands of times faster for a large bulk of data because SQL Server can choose an effective execution plan. However, if you like to use such variables and add a huge amount of data to them, you must continue reading.Įven if you replace the table variable with SQL, it will greatly speed up query performance: select * If there is little data in the table variable and you do not insert thousands of rows in it, everything is fine. In this case, the optimizer does not know how much and what data will be added to the variable, and whether there is sorting because such a variable does not include indexes. However, SQL Server chooses it when there is a lot of sorted data. It is much more effective to scan the whole Address table and find all the users at once. In terms of execution, due to the insane number of scans, the server simply drops trying to find data. If there are 1000 records in the server will scan Address 1000 times. Thus, for each ID from a search in the Address table will occur. If I’m not mistaken, the query optimizer will be always using the LOOP execution method. The fact is that table variables were not designed for processing large volumes of data. However, the performance of the query may suffer. In the C# code, you can quickly combine the results of both data arrays into one object using LINQ. Join Address a on a.UserID = users.UserIDĪll this works correctly. We got a bunch of user IDs from the service, and to avoid querying the database, someone decides that it’s easier to add all the IDs to the query parameter as a table variable and the query will look neatly: select * Perhaps, someone may ask why we do not execute one query on the database and get everything right away? I have a simple example.Īssume that users come from the Web service, while their addresses are stored in your database. Now, you collect IDs of all users, add them to the table variable and can search addresses for these users. For example, there is a query in the code that returns users of the site. Select * from noticed that table variables are used when it is necessary to fetch data for a large selection. We can add much data to it and perform data retrieval from this variable: insert into UserID Now, we can use this variable in our queries. It is possible to create more complex tables, however, in our example, one column is sufficient to explore the optimization. Here, we declare the variable as a table that will contain a single Value column of the Integer type. Thus, you can write the following: declare as table (int value) Perhaps, other databases have the same capabilities, however, I used such variables only in MS SQL Server. In SQL Server, we can create variables that will operate as complete tables. In this article, we are going to touch upon the topic of performance of table variables.
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