But if we were only looking at a few products this could really well. Often it is very easy to fix them and we can improve the performance of the server in just few minutes. Table variables are created like any other variable, using the DECLARE statement. In this article, I’ll explain the main factors involved in choosing one or the other, and demonstrate a few simple ‘rules’ to get the best performance. Temporary Table vs Table Variable While you can now use the inline index creation syntax to create non-clustered indexes on a table variable, there are some restrictions, and there are still no associated statistics. Let me paint a picture of the first scenario. But Session 1, which is above session 2, will not be able to see the SQL Server temp table. I will blog about it in the separate blog post. 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. CTE - Common Table Expressions. 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. Finally, you might be in a situation where you need the data to be visible only in the current session. SQL Table Variable Example. Table variables tend to get ‘bad press’, because queries that use them occasionally result in very inefficient execution plans. 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. Lots and lots of blocking. When they are used in stored procedures, there is less contention on system tables, under conditions of high concurrency. Aamir is a SQL Server Database Administrator in the NYC/NJ area (and has recently taken a role as a Database Developer). 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). We’ll start with an example where a table variable is ideal, and results in better performance. Here, it scans one table and then for each row returned performs individual seeks of the other table. Table variables are very simple to use, mainly because they are “zero maintenance”. He has rewritten a stored procedure using table variable instead of temp table, it makes the stored procedure code look more tidy. There are two important factors: if you have a result of over, let us say, 1000 rows (and this figure depends on context) then you need to have a PRIMARY KEY or UNIQUE key for any queries that join to a table variable. To create a global SQL temp table, you simply use two pound symbols in front of the table name. A table variable name must begin with an @ sign, such as @_my_first_table_variable. You can create a temp table with either a create table statement or the into clause in a select statement. CTE stands for Common Table Expressions. Finally, we run the same queries with temporary tables. Unlike Temporary Tables, they cannot be dropped explicitly. ⇒ Table variables cannot be involved in transactions, logging or locking. Also, on a final note, in terms of transactions on table variables. They are not visible outside of the batch, meaning the concept of session hierarchy can be somewhat ignored. It … Temp Tables and Temp Variables both support unique key, primary key, check constraints, Not null and default constraints but a Temp Variable doesn't support Foreign Keys. Lo, the story changes for them so that all three timings are much closer. Sometimes, but rarely, even this won’t help. Pros and cons of table variables and temporary tables Table variables tend to get ‘bad press’, because queries that use them occasionally result in very inefficient execution plans. You need to make sure to clear up after yourself, to avoid tempdb congestion. Phil Factor improves SQL Prompt's built-in ALTER TABLE ADD (ata) snippet so that it enforces certain coding standards, such as specifying whether the column accepts NULL values, and ensuring the new column is well-documented. Overall, the temp tables look to be the best choice, but we’re not finished yet! 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. What, you wonder, would happen if you gave those poor heaps the OPTION (RECOMPILE) hint too? At a certain point, the compromises that give them a better performance (not triggering recompiles, not providing statistics, no rollback, no parallelism) become their downfall. Not to say that one is more useful than the other, it’s just you have to choose the right tool for the job. Many believe that table variables exist only in memory, but that is simply not true. Generally speaking, we should choose temp tables where they work but this will not be the best choice in absolutely every circumstance. SQL temp tables are created in the tempdb database. See the code sample above, your loop can now reference the SQL Server temp table and process the records that meet the criteria of your goal. A consultant had attempted to justify his choice of using temp tables, as opposed to table variables, by declaring that temp tables are purely in-memory structures, incurring no physical disk I/O, and therefore perform as well as table variables. But this process is transparent to the developer/user. Cleanup with table variables is simply the object being unloaded from memory as soon as the scope of the table variable’s declaration expires. Active 2 years, 9 months ago. they have entries in the system tables in tempDB, just like temp tables, and they follow the same behaviour regarding whether they are in memory or on disk. ⇒ Temporary tables are allowed CREATE INDEXes whereas, Table variables aren’t allowed CREATE INDEX instead they can … You’ll see the queries shortly, when I show the code for the Test Rig. They are easier to work with and they trigger fewer recompiles in the routines in which they’re used, compared to using temporary tables. The sessions below it (sessions 3 and session 4) are able to see the SQL Server temp table. 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. If you’re working with a table variable heap, then you can only use it a simple list that is likely to be processed in a single gulp (table scan). They are scoped to the batch or routine in which they are created, and are removed automatically once it completes execution, and so using them within a long-lived connection doesn’t risk ‘resource hogging’ problems in tempdb. This is especially true if the data must be entirely removed from the table, as only temporary tables support truncation. Cleanup of Table Variables. The SQL temp table is dropped or destroyed once the session disconnects. |   GDPR   |   Terms of Use   |   Privacy. We have two object types each with their own strengths and weaknesses. Once it moved to production, the system screeched to a halt. This is a very simplified example, and we wouldn’t use it if it contained a lot of rows. So table variable is faster then temporary table. Global Temp tables are available to all sessions or connections. At a certain point, you will also need to trigger a recompile to get a decent execution plan, which has its own overhead. 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 : … If you alter a temporary table, or modify the data in them, you may incur recompiles of the parent routine. These give a great example of the ghastly performance you can experience if you don’t know the rules. There is nothing reckless about using table variables. Armed with correct row counts and ordered inputs, the optimizer chooses the far more efficient Merge Join. Interestingly, the two “common words in Dracula” queries that were fast even on heaps are now much slower. One of the most often used scenarios for SQL Server temp tables is within a loop of some sort. Lastly, we are selecting all the records from Table Variable, whose Total Income is … Also note, that in the event of name conflict (remember that SQL Server temp tables are created in the tempdb) SQL server will append a suffix to the end of the table name so that it is unique within the tempdb database. So there is a lot of flexibility and allows the developer to be quite creative. You can use your existing read access to pull the data into a SQL Server temporary table and make adjustments from there. The estimated plans looked the same using temp table and table variable… 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. You will need to remove the OPTION (RECOMPILE) hints to get back to the original. 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. : Sql server table variable vs. local temporary table. Viewed 478 times 0. Temp table operations are carried out as part of the user transaction itself, whereas table variable operations are carried out as system transactions, independent of any outer user transaction. The persistence difference means you should ‘drop’ (delete) the table when you are finished which is an added step compared to variable tables. The results I’ve shown you in this article will suggest to you that this oversimplifies the issues. Also note that anyone can DROP the table. Also like local SQL temp tables, table variables are accessible only within the session that created them. Here are a few blog posts which are discussing the same topic in detail: This is a guest post from Phil Factor. 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 … 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. This article describes a comparison on SQL temp table and table variable.SQL offer four types of table structure to store data. Without the metadata that they provide, the optimizer has no knowledge of the logical order of the data, or whether the data in the join column contains duplicate values, and will likely choose inefficient join operations, resulting in slow queries. The experienced developer or DBA will be on the lookout for this sort of problem, and be ready to add the OPTION (RECOMPILE) query hint to the statement that uses the table variable. He has worked for various industries in both large and small environments all with different needs. 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. When working with relatively small data sets, they are faster than the comparable temporary table. For the tables variable with primary keys, the optimizer knows the order of the rows in join column, and that they contain no duplicates, but assumes it’s only dealing with one row, and so chooses instead a Nested Loops join. The more rows in the table variable, the larger the discrepancies between estimation and reality, and the more inefficient become the optimizer’s plan choices. The Nested Loops join reaches ‘peak inefficiency’ for two, ten-minute queries using table variable heaps, because it entails thousands of table scans of CommonWords. When we submit a batch containing a table variable, the optimizer first compiles the batch at which point the table variable is empty. In fact, the table variable provides all the properties of the local variable, but the local variables have some limitations, unlike temp or regular tables. 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). Ask Question Asked 2 years, 9 months ago. 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. In this quick diagram, a SQL temp table is created in Session 2. There are 60,000 common words, but Bram Stoker only used 10,000 of them. Many times you’ll see developers use the “DROP #Table_Name” command at the end of their statement just to clean up. They can be declared in batch or stored procedure. A Table Variable will always have a cardinality of 1, thus statistics are not tracked for them and may result in bad query plan. Also, over-reliance on this hint will negate to some extent the advantage that table variables have of causing fewer recompiles than temporary tables. Global SQL temp tables are useful when you want you want the result set visible to all other sessions. We saw two reasons for using table variables rather than temp tables. Following are the results of the initial test runs. The puzzling part is the same stored procedure now running a lot slower. Let’s try it out. The larger the row count and the index key, the more the difference increases. This makes @table faster then #temp. It doesn’t know that there are no duplicates, so in fact it doesn’t aggregate down at all, and the aggregation and subsequent join spill to tempdb. Here’s a quick example of taking a result set and putting it into a SQL Server temp table. 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. … Even the indexes that enforce PRIMARY KEY and UNIQUE constraints on table variables do not have statistics. 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. Here is a quick example of setting up and using a table variable. Often, the SQL Server pundit will give sage advice about the size of result that will cause problems for a table variable. Because the optimizer in some cases may ignore the number of records in table variable while generating the query plan. What is the difference between Clustered and Non-Clustered Indexes in SQL Server? They are also not fully logged, so creating and filling them is faster and requires less space in the transaction log. 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. We’ve created a table variable that will hold information regarding total quantities of a certain product sold. People can, and do, argue a great deal about the relative merits of table variables and temporary tables. Aamir is a SQL Server Database Administrator in the NYC/NJ area (and has recently taken a role as a Database Developer). E.g. For larger table variables, nonclustered indexes use more memory than they do for memory-optimized tables. Disk I/O in tempdb as @ _my_first_table_variable current session have statistics but session 1, which they. 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