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Today we will talk about optimization techniques for temporary tables

Temporary table, as the name implies, is a table that is temporarily used. One is a local temporary table, which can only be used on the current query page. It cannot be used for newly opened queries. The other is a global temporary table, no matter how many query pages are opened. be usable.

1. Local temporary table

The local temporary table can add # in front of the table name, let's take a look at the characteristics of the local temporary table

We create a new query page and enter the following code:

SELECT TOP 10 * INTO #temp
FROM sales.Temp_Salesorder;
SELECT * FROM #temp;

The results are as follows:

Let's open a new page and re-enter the following code:

SELECT * FROM #temp;

The results are as follows:

Prove that local temporary tables can only be executed on the current page.

2. Global temporary table

The global temporary table can be added with ## before the table name, and it can be used when opening any query page.

Repeat the above steps:

SELECT TOP 10 * INTO ##temp
FROM sales.Temp_Salesorder
SELECT * FROM ##temp;

The result is the same as above:

We open a new page:

SELECT * FROM ##temp;

The result is still the same. Prove that all query pages of the global temporary table can be used.

3. The optimization method of temporary table

After introducing the temporary table, let's talk about how to use it to optimize

The optimization of temporary tables is generally called nested queries when there are many sub-queries. We write the following subquery:

SELECT * FROM sales.Temp_Salesorder
WHERE SalesOrderDetailID IN 
(SELECT SalesOrderDetailID FROM sales.SalesOrderDetail
WHERE UnitPrice IN
 (SELECT UnitPrice FROM sales.SalesOrderDetail WHERE UnitPrice>0)
)

(Hint: the code can slide left and right)

This is a relatively simple two-level nested subquery, let's take a look at the implementation:

It can be seen that the logical read here is relatively high.

Let's use the temporary table to re-look at the implementation. We insert the first and second layer query results into #temp, and then query the results from the temporary table.

SELECT SalesOrderDetailID INTO #temp FROM sales.SalesOrderDetail
WHERE UnitPrice IN (SELECT UnitPrice FROM sales.SalesOrderDetail WHERE UnitPrice>0)

SELECT * FROM sales.Temp_Salesorder
WHERE SalesOrderDetailID IN 
(SELECT SalesOrderDetailID FROM #temp)

The implementation is as follows:

Compared with the last logical read, the number of logical reads is reduced by a factor of two.When adjusting the performance of a query, if the logical read value drops, it indicates that the server resources used by the query decrease and the performance of the query improves. If the logical read value increases, it means that the adjustment measures reduce the performance of the query. In the case of other conditions unchanged, the fewer logical reads a query uses, the higher its efficiency and the faster the query.

Therefore, we can see that temporary tables can improve query efficiency in more complex nested queries.

That’s all for today’s class. If you don’t understand, you can leave a message below. I will reply one by one.

——End——

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