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−.. This Source Code Form is subject to the terms of the Mozilla Public
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−.. License, v. 2.0.  If a copy of the MPL was not distributed with this
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−.. file, You can obtain one at http://mozilla.org/MPL/2.0/.
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−..
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−.. Copyright 2013-2021 MonetDB B.V.
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−
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−.. This document is written in reStructuredText (see
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−   http://docutils.sourceforge.net/ for more information).
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−   Use ``rst2html.py`` to convert this file to HTML.
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−
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−===================================
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−Implementing User-Defined Functions
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−===================================
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−
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−:Author: Sjoerd Mullender
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−:Organization: CWI, MonetDB B.V.
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−:Abstract: In this short tutorial we describe how to create a simple
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−   UDF for MonetDB/SQL.
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−
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−Introduction
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−------------
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−
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−In this directory there are a number of tutorials for creating
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−user-defined functions.  It is recommended to go through these
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−tutorials in the order presented here, since they will go from simple
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−to more complicated.
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−
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−These tutorials are about extending MonetDB/SQL using one or more
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−functions written in C.  There are other ways in which to extend the
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−functionality of MonetDB/SQL that may be easier to implement.  See
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−`Embedded Python/NumPy in MonetDB`__ and `Embedded R in MonetDB`__.
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−
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−__ https://www.monetdb.org/blog/embedded-pythonnumpy-monetdb
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−__ https://www.monetdb.org/content/embedded-r-monetdb
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−
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−Setting Up
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−----------
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−
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−These tutorials work completely outside the source of the MonetDB
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−suite.  You do not need to copy the source of MonetDB, and you do not
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−need to recompile the MonetDB sources.  You do need to install some
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−extra packages that are not normally installed.
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−
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−On Fedora and RedHat-based systems you need to install the packages
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−``MonetDB-SQL-server5-devel``, ``MonetDB5-server-devel``,
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−``MonetDB-devel``, and ``MonetDB-stream-devel``.  On Debian and Ubuntu
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−you need to install the packages ``monetdb5-sql-dev``,
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−``monetdb5-server-dev``, ``libmonetdb-dev``, and
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−``libmonetdb-stream-dev``.  These packages are available from the
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−`MonetDB download site`__.
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−
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−__ https://dev.monetdb.org/downloads/
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−
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−Tutorials
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−---------
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−
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−reverse
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−.......
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−
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−Start with the tutorial in the ``reverse`` subdirectory.  In this
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−tutorial you will create a simple user-defined function (UDF) that
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−takes one (string) value as input and produces one (string) value as
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−output.
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−
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−regexp
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−......
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−
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−The tutorial in the ``regexp`` subdirectory contains an example of a
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−FILTER FUNCTION.  These functions are used to filter values in a
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−column.  The simplest example of a filter is (given a table ``t`` that
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−has an integer column ``c``):
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−
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−.. code-block:: sql
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−
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−  SELECT * FROM t WHERE c = 0;
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−
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−This query filters the table t and selects all rows where the value of
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−column ``c`` is equal to ``0``.
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−
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−gmean
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−.....
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−
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−The tutorial in the ``gmean`` subdirectory contains an example of an
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−AGGREGATE FUNCTION.  These functions are used to calculate aggregates
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−over columns of values.  Aggregates can be simple or grouped.