How to Retrieve a Single Column Using Dexie.js

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The Problem

Suppose you have a query that fetches data from an IndexedDB database. Your current fetch query might look something like this:

[[See Video to Reveal this Text or Code Snippet]]

This results in an array of objects, with each object representing a row of data, like so:

[[See Video to Reveal this Text or Code Snippet]]

In this case, suppose you are only interested in the sample property. The challenge is how to minimize the size of the result and get a simple array containing only the values of the sample field instead of entire object rows.

The Solution

Using a Compound Index

Define the Index:
When you initialize your database version, make sure to define a compound index that includes both the ts (timestamp) and sample fields.

[[See Video to Reveal this Text or Code Snippet]]

Query the Data:
Update your query to use the new compound index you created.

[[See Video to Reveal this Text or Code Snippet]]

This query will return an array of tuples that include both the timestamp and sample values:

[[See Video to Reveal this Text or Code Snippet]]

Mapping to Get Only the Sample Values

Now that you have the tuples, you can easily map them to extract just the sample values using the following code:

[[See Video to Reveal this Text or Code Snippet]]

Alternative Approach Without Compound Index

If you prefer to avoid creating a compound index for some reason, you can simply process the entire object array later. Although this might have a minor impact on performance, it works effectively if you're less concerned about optimized performance.

For example, after retrieving the original data:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Remember to consider your application's performance requirements when deciding whether to use compound indexes or simply to map over full objects to extract desired values.

With these techniques, you can optimize your data queries and focus on extracting only the necessary information from your database!