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Here at Jampp we process and analyze large amounts of data. One of the tools we employ to do so is PrestoDB, which is a “Distributed SQL Query Engine for Big Data”. Presto comes with many native functions, which are usually enough for most use cases. Nevertheless, sometimes you need to implement your own function for a very specific use.

Enter the User Defined Functions (UDFs, for short). Writing one for the first time is not as straightforward as it may appear, mainly because the information to do so is very scattered around the web (and across many Presto versions).

In this blogpost, we present our JSON_SUM function, how we wrote it, and some of the lessons we learned along the way. …

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