Powershell Perfomance with Where-Object? - performance

Question

See below code, the only difference in the two scripts is the lack of a script block in the second one, and there is a significant % performance increase due to it.
Is there a reason for this? Is one more native to powershell than the other?
I am doing a large number of scripts with multiple similar blocks in many of them, and would like a reasonable answer as to how to gain easy performance boosts such as this one, so why does the exclusion of a scriptblock in the Where-Object (alias ?) suddenly cut performance by a decent margin?
PS C:\Scripts> $a = 1..15 | % {
Measure-Command {
$G = Get-ADGroup -Filter *
1..3 | % {
$G | ? {$_.Name -eq "TestGroup$($_)"}
}
}
}
$b = 1..15 | % {
Measure-Command {
$G = Get-ADGroup -Filter *
1..3 | % {
$G | ? Name -eq "TestGroup$($_)"
}
}
}
($a.TotalMilliseconds | Measure -Average).Average
($b.TotalMilliseconds | Measure -Average).Average
283.479413333333
212.57384

Answer 1

PowerShell 3.0 introduced Where-Object (aliases: Where, ?) that can check properties directly without a scriptblock.
When a scriptblock is executed PowerShell, like any other language, creates a new execution context and it's very expensive in an interpreted language.
The only reasons to use the old notation with a scriptblock are:
to have a PS1/PS2-compatible code;
to perform complex checks;
to do something besides the check itself.
As for your code, both snippets use scriptblocks in other places, pipelining (it's 5-10x slower than foreach statement), and needlessly measure Get-ADGroup time, so we can optimize it even more:
$group = Get-ADGroup -Filter *
$c = foreach ($i in 1..15) {
Measure-Command {
foreach ($j in 1..3) {
$filtered = foreach ($g in $group) { if ($g.Name -eq "TestGroup$($j)") { $g } }
}
}
}
Sometimes a much bigger gain can be achieved by preparing the data. For example, if some array of objects is repeatedly checked in a loop to see whether it has a certain property value, it's better to convert the array into a hashtable with a key based on that property value:
# index by somefield
$lookup = #{}
foreach ($element in $array) { $lookup[$element.somefield] = $element }
# use the indexed lookup table
foreach ($element in $anotherarray) {
$existingElement = $lookup[$element.somefield]
if ($existingElement) {
# do something
}
}