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Comparing Optimization of Filtering in PowerShell

This morning in my Hunt Valley, MD PowerShell class, I extended yesterdays lesson (see yesterday’s post) into filtering optimization.  In the PowerShell world, we have a saying: “Filter to the Left”.  That means that you filter out as many objects as possible as close to the beginning of the piped commands as possible.  What we did was use the Get-EventLog cmdlet and filtered it in two ways.  We wanted to filter for Event ID 12.  In the Get-EventLog cmdlet, we used the InstanceID parameter with a value of 12.  In the second execution of Get-EventLog, we piped everything to Where-Object and filtered on the property InstanceID for a value of 12.  We then executed our code from yesterday to test the runtime for each one.

# Optimizing for Performance.

# Get-Help Get-EventLog -Parameter Newest


# Execute each section individually by highlighting

# the code and pressing F8.



# This is optimized

Get-EventLog -LogName System -InstanceId 12


# This is not Optimized

Get-EventLog -LogName System |

Where-Object InstanceID -eq 12



# Get the history information and execution times.

Get-History |

    Select-Object -Property CommandLine,


        E={($_.EndExecutionTime - $_.StartExecutionTime).TotalSeconds}} |

    Select-Object -Last 2

Here is the output from the fourth section.

CommandLine                                                                  ExecutionTime

-----------                                                                  -------------

Get-EventLog -LogName System -InstanceId 12                                      8.9346474

Get-EventLog -LogName System |...                                               14.6560631

You can see that the first command using its built in filtering capabilities is much faster than piping all the objects to Where-Object.


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