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FUNCTIONS.md

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+# Presto Function Guidelines
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+Presto includes a large library of built-in, core SQL functions such as AVG and CONCAT that are available in every deployment.
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+Presto also includes extension functions that are not enabled by default but can be optionally enabled in particular deployments.
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+Finally, Presto allows users to write user defined functions (UDFs) in either Java or SQL that are not stored in this GitHub repository. This is done through the [Plugin SPI](https://prestodb.io/docs/current/develop/functions.html).
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+This document lays out some guidelines for which category is right for which functions. 
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+Functions defined in [ANSI SQL](https://jakewheat.github.io/sql-overview/) should be written as core functions in this repository.
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+Non-standard functions that are commonly available in multiple other SQL dialects such as MySQL and Postgres should also be written as core functions in this library.
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+Core functions should be broadly applicable to many users in different industries.
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+Core functions should be purely computational. That is, they do not make network connections or perform I/O outside the database. (Date/time functions like NOW() are an exception.) For example, a function that talks to a remote server to check the current  temperature or a stock price should not be a core function. This should be a user defined function maintained outside the Presto GitHub repository. 
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+Functions that alias or closely resemble existing core functions should not be included in this repository. For instance, because Presto already has an AVG function it should not also have a MEAN or an AVERAGE function, even if the semantics or arguments are slightly different.
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+Functions that include trademarked names should be UDFs outside this repository.