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Home > SQL, SQL Server > Understand Response Time vs. Total Time in SQL

Understand Response Time vs. Total Time in SQL

Response time is the time it takes for a query to return the first record. Total time is the time it takes for the query to return all records. For an interactive application, response time is important because it is the perceived time for the user to receive visual affirmation that a query is being processed. For a batch application, total time reflects the overall throughput. You have to determine what the performance criteria are for your application and queries, and then design accordingly.

For example, suppose the query returns 100 records and is used to populate a list with the first five records. In this case, you are not concerned with how long it takes to return all 100 records. Instead, you want the query to return the first few records quickly, so that you can populate the list.

Many query operations can be performed without having to store intermediate results. These operations are said to be pipelined. Examples of pipelined operations are projections, selections, and joins. Queries implemented with these operations can return results immediately. Other operations, such as SORT and GROUP-BY, require using all their input before returning results to their parent operations. These operations are said to require materialization. Queries implemented with these operations typically have an initial delay because of materialization. After this initial delay, they typically return records very quickly.

Queries with response time requirements should avoid materialization. For example, using an index to implement ORDER-BY yields better response time than does using sorting.

The following section describes this in more detail.

Index the ORDER-BY / GROUP-BY / DISTINCT Columns for Better Response Time

The ORDER-BY, GROUP-BY, and DISTINCT operations are all types of sorting. The SQL Server Compact 3.5 query processor implements sorting in two ways. If records are already sorted by an index, the processor needs to use only the index. Otherwise, the processor has to use a temporary work table to sort the records first. Such preliminary sorting can cause significant initial delays on devices with lower power CPUs and limited memory, and should be avoided if response time is important.

In the context of multiple-column indexes, for ORDER-BY or GROUP-BY to consider a particular index, the ORDER-BY or GROUP-BY columns must match the prefix set of index columns with the exact order. For example, the index CREATE INDEX Emp_Name ON Employees (“Last Name” ASC, “First Name” ASC) can help optimize the following queries:

  • … ORDER BY / GROUP BY “Last Name” …
  • … ORDER BY / GROUP BY “Last Name”, “First Name” …

It will not help optimize:

  • … ORDER BY / GROUP BY “First Name” …
  • … ORDER BY / GROUP BY “First Name”, “Last Name” …

For a DISTINCT operation to consider a multiple-column index, the projection list must match all index columns, although they do not have to be in the exact order. The previous index can help optimize the following queries:

  • … DISTINCT “Last Name”, “First Name” …
  • … DISTINCT “First Name”, “Last Name” …

It will not help optimize:

  • … DISTINCT “First Name” …
  • … DISTINCT “Last Name” …
If your query always returns unique rows on its own, avoid specifying the DISTINCT keyword, because it only adds overhead

Thanks
Yashwant

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