When dealing with large data sets in PowerShell, we usually use Where-Object to filter collections based on some conditions. However, when the collections become large, the execution can slow down, causing a bottleneck and impacting the performance of our scripts. There is an alternative approach - the use of lookup tables. Let's dive into it using a real-life example.
The Case Study: Filtering Users
A lot of my consultant work is correlating data across multiple systems, for example, HR data with disparate directories. For our test, let's assume we have a list of 10,000 users. Each user is an object with properties like FirstName, LastName, Email, Username, EmployeeId, Department, and Title. First, let's create a dummy list:
Suppose we have another set of data where we need to extract data from users that match on EmployeeId.
Probably the most intuitive way would be to loop through each employee and perform Where-Object on the users' collection attempting to match using EmployeeId from the employees collection. Like this:
This operation, while simple, will take quite some time to execute on a large collection like ours, as Where-Object will have to scan through each object one by one. Let's measure the command for comparison's sake:
Close to 2 minutes. Not the end of the world, but we can do better.
The Alternative: Lookup Tables
A lookup table, in essence, is a hashtable that we can use to quickly find corresponding values by their keys. This approach can significantly speed up operations where we need to match objects based on a common property. Let's use a lookup table to achieve the same result as before. First, let's create one:
In the code above, we create a lookup table where each key is an EmployeeId, and the value is a list of user objects. The $lookup_table[$employee.EmployeeId] operation is near instantaneous. Now let's loop through each employee and measure the command using the lookup table. I'll include the creation of the lookup table in the measurement.
Less than a second. I think that's an improvement, no?
Wrapping Up
While Where-Object remains a fundamental tool in PowerShell scripting, lookup tables often offer superior performance when dealing with larger data sets. With that said, they are not without limitations. They are best suited for situations where you have unique key-value pairs and need frequent lookups. If the keys aren't unique or you're dealing with small data sets, the initial overhead of creating the lookup table may not justify the performance gain. Even considering their constraints, employing lookup tables can significantly enhance the performance of your scripts in the right circumstances.
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