SQL Server Logical Reads – What do they really tell us?

SQL Server trace, the most
common tool DBAs use to evaluate query performance, provides the ‘logical
reads’ counter on which many DBAs rely for evaluating a query’s I/O
performance. In this article, we will examine this counter’s true meaning and
provide examples that prove it can sometimes be quite misleading…

I am sure you have all used SQL traces to evaluate the
performance of queries and batches. The most common data columns used for this
purpose are Duration, CPU, Writes and Reads. These
are in fact the only true performance metrics available for a SQL Server trace
event. A common misconception I’ve encountered in talking with numerous DBAs is
that “reducing the number of reads a query performs is an important aspect
of improving its performance”
. Although this may prove to be true in many
cases, in this article I want to draw your attention to the fact that “it
ain’t necessarily so” ? sometimes the opposite is true.

First, we need to understand what a Read really is. Here
is a quote from a Microsoft white paper about I/O architecture that clearly defines
logical and physical reads: “The I/O from an instance of SQL Server is
divided into logical and physical I/O. A logical read occurs every time the
database engine requests a page from the buffer cache. If the page is not
currently in the buffer cache, a physical read is then performed to read the
page into the buffer cache. If the page is currently in the cache, no physical
read is generated; the buffer cache simply uses the page already in memory.”

It is important to remember that SQL Trace reads are logical
reads and not physical reads. I assume that this has to do with the fact that
physical reads are common to all currently (and recently) executing batches on
the server and therefore are not truly assigned to a specific event, even
though they are always triggered by a specific event. Depending on the current
content of the cache, the same query or batch may or may not trigger a physical
read. Physical reads are the real cause of pain as far as performance is
concerned because storage subsystems are unfortunately (still) inherently slow,
as opposed to logical reads that occur in the super-fast modern memory modules.

* Physical
reads metrics are available when using STATISTICS IO and from SQL Server DMVs.

I think the best way to make my point is by using an
example. For this demo I have used SQL Server 2005 SP3 and the “AdventureWorks
sample database.

* As
we cannot know in advance what the cache content will be at any given point in
time,  for the example code, I’ve explicitly flushed the cache to simulate the
worst case scenario of an empty cache.

Let’s take the following
example query and assume I was given the task of optimizing its performance:

SELECT      C.CustomerID, SOH.SalesOrderID, SOH.OrderDate

FROM        Sales.Customer C

            INNER JOIN

            Sales.SalesOrderHeader SOH

            ON SOH.CustomerID = C.CustomerID

WHERE       C.TerritoryID
= 1 AND C.CustomerType = N’S’

The metrics for this query recorded on my PC using profiler
are as follows:

Note:
Numbers below indicate averages of several executions, using a cold cache.

CPU: ~40, Reads: ~840, Duration ~300ms.

The “missing indexes” option in management studio suggested
that I add the following index:

/*

Missing Index Details from logical_reads.sql -
AMI-PC.AdventureWorks (DBSOPHICAmi (52))

The Query Processor estimates that implementing the following
index could improve the query cost by 13.1751%.

*/

/*

USE [AdventureWorks]

GO

CREATE NONCLUSTERED INDEX [<Name of Missing Index,
sysname,>]

ON [Sales].[Customer] ([TerritoryID],[CustomerType])

INCLUDE ([CustomerID])

GO

*/

Being an obedient DBA, I immediately obliged, created this
index and then executed the same query again. This time, I got the following
performance metrics in profiler:

CPU: ~20, Reads: ~4000, Duration ~200ms.

What happened here? Did adding this index improve the
performance of the query or worsen it? It seems I have some conflicting
information here. CPU and duration seem to have improved significantly but the
logical reads have increased by nearly a factor of 5!

To understand what’s going on behind the scenes, we must
consult the execution plan.

This is the original execution plan of the query before the
suggested index was created:

And this is the plan the
optimizer chose after the suggested index was created:

In the original plan, the optimizer chose to join the tables
using a hash match physical operator. In a hash match, each value of customerID
from each of the joined tables needs to be accessed only once. The hash key
buckets were built on the CustomerID values from customer’s table
and then probed by scanning the SalesOrderHeader table. Each page needed
to be read only once and all values were retrieved. You can verify this by
examining the number of pages used by both tables – the Customer table
is ~110 pages large and the SalesOrderHeader table is ~700 pages. This
accounts for the ~800 reads we saw in the trace. It is also important to
remember that the hash table was probed 31,465 times – once for each key from
the SalesOrderHeader table. These probes, which do consume resources, do
not constitute a logical read and are not available as separate counters in the
SQL Trace and in STATISTICS IO.

After the index was created, the optimizer had many more options
to work with. The index on the customers table allowed efficient seek
filtering on both the TerritoryID and CustomerType predicates,
resulting in less than a 100 rows that satisfy the filters. The optimizer
decided (correctly) that performing a nested loops operator, retrieving the
pointers to all relevant rows from the customer table, and then
performing a key lookup to retrieve the OrderDate and OrderID
columns for the select list would be more efficient. Now, because each row had
to be ‘index seeked’ individually, the same pages needed to be accessed in
memory multiple times, each counting as a logical read. As a result, the total
number of logical reads increased significantly.

 Overall, the query performance significantly improved in
several aspects:

·        
Only the correct subset of rows from the customers table was
retrieved, potentially reducing physical IO and locking contention

·        
The CPU intensive hash functions were eliminated

·        
The memory footprint for both data and hash buckets was reduced

This example clearly demonstrates that when evaluating a
query’s performance, you must always consider all aspects of its execution and
remember that the logical reads metrics might be highly misleading in some
cases. Flushing the buffer cache and using STATISTICS IO instead will provide
you with the physical reads counter which, in many cases, more accurately
reflects the I/O demands of the analyzed query.

One final thought: Although no more index recommendations
were suggested by the query optimizer, can you think of an additional index
that could improve this query’s performance even more? Take a look at the attached
demo code to see the additional index I suggested and
its effect on the query’s performance.

p.s. Try to execute the demo code on SQL 2008 and you are in
for a surprise…
More on that, in the next article.

_______________________________________

Ami Levin
is a Microsoft SQL Server MVP, with over 20 years experience in the IT
industry. For the past 12 years, he has been consulting, speaking and teaching
SQL worldwide. He manages the Israeli SQL Server user group, leads the SQL
Server support forum in Hebrew, and is a regular speaker at Microsoft
conferences. Ami is the CTO and co-founder of DBSophic, a software company that develops
innovative solutions for performance optimization of SQL Server application
workloads.




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