8. Data Sampling Video

eTrainerbox

5/9/2025

Transcript

00:00Let's dive into the topic of data sampling.

00:03To optimize performance, Tableau Prep samples large datasets and returns a subset of records.

00:09The subset of rows returned from your data sources may or may not be representative of your data,

00:15depending on the settings that you've chosen.

00:18Your sample may be optimized for speed, but not necessarily a representative sample of your data.

00:24So it's important that you make sure your settings are to your liking,

00:28and if they aren't returning the right results, you can change them right within the input step.

00:33Let's take a quick look at the different options for data sampling.

00:37Now you can see the data sampling options inside your data sample tab in your input step.

00:43Once you click on the data sample tab, you'll have two main sections to be concerned about.

00:47You can select the amount of data to include in the flow, and you can choose the sampling method.

00:53Now the default sample amount is chosen by PrepBuilder.

00:57PrepBuilder will determine the number of rows to return for your dataset.

01:01This is an automatic algorithm that Tableau Prep uses to optimize the data sample.

01:07Alternatively, you can also choose to use all data to run through your flow.

01:12This will retrieve all rows regardless of the size of your dataset.

01:16And as a result, this can cause performance issues.

01:20Note that even if you pick use all data, there will be some limitations.

01:24Data will still limit to 1 million rows or less for the aggregate and union steps,

01:30and 3 million rows or less for the join and pivot steps.

01:35Now this doesn't mean that the data will be limited when you actually go and run your flow.

01:40It just means that when you're building the workflow, it's going to limit the amount of data that it uses to profile your datasets.

01:48That means when you're filtering, aliasing, and doing other cleaning steps, you may be missing data depending on which option you're choosing.

01:56You can also choose a fixed number of rows to pull.

02:00It's recommended that this is less than a million records for performance reasons.

02:05Now when we talk about sampling method, QuickSelect is the default.

02:10Using QuickSelect, a sample is returned as quickly as possible.

02:14It will use n number of rows or cached data that's available from a prior query to develop your sample.

02:22Now this option is less accurate but often quicker than the random sample.

02:27The random sample will return the number of rows requested,

02:31but it will look at all records and return a representative sample of data.

02:36Now using a random sample may impact performance, but this will only be before your standard cache.

02:43Once you run your flow more than once, some of the data will cache in your memory,

02:47and you'll be able to run the flow again with faster processing time.

02:52Now that we've looked at the data sampling options,

02:54let's jump into Tableau Prep and choose data sampling options for our own datasets.

02:59Okay, so we're back in Tableau Prep, and we're going to choose data sampling for all of our inputs.

03:05For our school level inputs, we're going to choose random samples,

03:09and for our district level inputs, we're going to use all data.

03:13Now technically, our data is not huge, so we'll really be pulling all of our dataset through our flow anyway,

03:21but this will get you some good practice at setting your data sampling.

03:25Let's go ahead and click Plan of High School Grads.

03:28We'll pull this up so we can see it a little bit better.

03:31Choose Data Sample, and we're going to choose a random sample for our sampling method.

03:36Again, this will be a more thorough sample than the Quick Select,

03:41but that won't matter too much for performance in our place because our datasets are not very large.

03:47You will want to consider this for datasets that are large, however, as it could impact performance.

03:53Let's go to our other school inputs and change to random sample.

03:57So we'll click on Educator Evaluation Performance.

04:02We'll go to Data Sample, and we'll choose Random Sample.

04:06Then we'll go into our SAT Performance.

04:09We'll go to our Data Sample tab, choose Random Sample here.

04:13We'll go to Teacher Data, click Data Sample, and click Random Sample here.

04:19Alright, so that's all of our school inputs, these top four,

04:22and then the bottom four are going to be our inputs at district level.

04:26So we'll click on Per Pupil Expenditures.

04:30We'll click Data Sample, and instead of Random Sample here,

04:35we're going to want to modify our select amount of data to include in the flow.

04:40Now for these datasets, we're going to want to choose Use All Data.

04:44Now all of the data will be used in our flow.

04:47You'll notice how sampling method is grayed out.

04:50That's because you're not sampling if you're pulling in all of the data.

04:53Sampling is a subset of your total dataset, but if we're using our total dataset,

04:58there's no need for sampling.

05:00Let's go ahead and click on our Advanced Course Data Source,

05:04choose Data Sample, and choose Use All Data.

05:08And we'll go to our Teacher Salaries dataset, click Data Sample, and choose Use All Data.

05:15And up next, we'll talk about how to refresh your data input connections.

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