Lua

This article explains how to use Lua 5.4 for UDF scripting in Exasol.

For additional guidelines on using Lua with Exasol, see the Exasol Lua Style Guide.

For more information about the Lua language, refer to the official Lua documentation.

run() and cleanup() Methods

The method run() is called for each input tuple (SCALAR) or each group (SET). Its parameter is a kind of execution context and provides access to the data and the iterator in case of a SET script.

To initialize expensive steps (for example, opening external connections), you can write code outside the run() method, this code is executed once at the beginning by each virtual machine. For deinitialization purpose, the method cleanup() is called once for each virtual machine at the end of the execution.

Parameters

The internal Lua data types and the database SQL types are not identical. Therefore, casts must be done for the input and output data:

The decimal type is not a standard Lua data type. For more details about the Lua data types, see General Script Language.

NULL values are represented by the special constant NULL.

The input parameters can be addressed by their names, for example, ctx.my_input. ctx refers to the name of the context parameter that is passed to the run() method.

However, you can also use a dynamic number of parameters through the notation (...), for example, CREATE LUA SCALAR SCRIPT my_script (...). The parameters can then be accessed through an index, for example, ctx[1]. The number of parameters and their data types (both determined during the call of the script) are part of the metadata.

Metadata

The following metadata can be accessed via global variables:

Data iterator

For scripts having multiple input tuples per call (keyword SET), you can iterate through that data using the method next(). The method is accessible through the context. Initially, the iterator points to the first input row. Therefore, a repeat...until loop can be used to iterate through the data (see examples).

If the input data is empty, then the run() method will not be called, and similar to aggregate functions, the NULL value is returned as result for example, SELECT MAX(x) FROM t WHERE false.

The method reset() resets the iterator to the first input element. If required for your algorithm, you can use it to do multiple iterations through the data.

The method size() returns the number of input values.

emit()

You can return multiple output tuples per call (keyword EMITS) through the method emit().

The method expects as many parameters as output columns were defined. In the case of dynamic output parameters, use an array using unpack(), for example, ctx.emit(unpack({1,"a"})).

Import other scripts

You can import the other scripts through the method exa.import(), for example as shown below.

Syntax

exa.import("<schema>.<script>", "<alias>")

Example

--/
CREATE OR REPLACE LUA SCALAR SCRIPT LIB.MYLIB() RETURNS INT AS
function helloWorld()
  return "Hello Lua World!"
end
/

--/
CREATE OR REPLACE LUA SCALAR SCRIPT TEST.MYHELLOWORLD() 
   RETURNS VARCHAR(2000) AS

exa.import("LIB.MYLIB","MYLIB")

function run(ctx)
  return MYLIB.helloWorld()
end
/

select TEST.MYHELLOWORLD();

You can also import scripting programs, however, their parameters will be ignored.

Access connection definitions

The data that has been specified when defining connections with CREATE CONNECTION is available in LUA UDF scripts through the method exa.get_connection("<connection_ name>"). The result is a Lua table with the following fields:

Auxiliary libraries

The following table lists the supported libraries:

For more information about the libraries, see Libraries.

The modules math, table, string, and unicode.utf8 are already provided in the namespace. You can load the others through require().
You cannot install additional Lua libraries.

Dynamic output parameters callback function

If the UDF script is defined with dynamic output parameters and the output parameters cannot be determined (by specifying EMITS in the query or through INSERT INTO SELECT), you can implement the database call to the function default_output_columns(). The expected return value is a String with the names and types of the output columns, "a int, b varchar(100)". For an explanation of when the method is called, see Dynamic input and output parameters. The method will be executed only once on a single node.

You can access metadata using exa.meta variables. For example, to find out the number and types of input columns.

User defined import callback function

To support a user defined import you can implement the callback method generate_sql_for_import_spec(import_spec). For syntax, see Dynamic input and output parameters and the IMPORT statement. The parameter import_spec contains all information about the executed IMPORT FROM SCRIPT statement. The function has to generate and return a SELECT statement which will retrieve the data to be imported. For more information about using UDFs in IMPORT statements, see IMPORT.

import_spec is a Lua table with the following fields:

If a password was specified, it will be transferred into clear-text and might be shown in the logs. It is recommended to create a CONNECTION before and to specify only the connection name (can be obtained from the connection_name field). The actual connection information can be obtained through exa.get_connection(name).

User defined export callback function

To support a user defined export you can implement the callback method generate_sql_for_export_spec(export_spec). For syntax, see Dynamic input and output parameters and the EXPORT statement. The parameter export_spec contains all information about the executed EXPORT INTO SCRIPT statement. The function has to generate and return a SELECT statement which will generate the data to be exported. The FROM part of that string can be a dummy table (for example, DUAL) since the export command is aware which table should be exported. But it has to be specified to be able to compile the SQL string successfully. For more information about using UDFs in EXPORT statements, see EXPORT.

export_spec is a Lua table with the following fields:

If a password was specified, it will be transferred into clear-text and might be shown in the logs. It is recommended to create a CONNECTION before and to specify only the connection name (can be obtained from the connection_name field). The actual connection information can be obtained through exa.get_connection(name).

Example

/*
  This example loads from a webserver
     and processes the following file goalies.xml:

     <?xml version='1.0' encoding='UTF-8'?>
     <users>
         <user active="1">
             <first_name>Manuel</first_name>
             <last_name>Neuer</last_name>
         </user>
         <user active="1">
             <first_name>Joe</first_name>
             <last_name>Hart</last_name>
         </user>
         <user active="0">
             <first_name>Oliver</first_name>
             <last_name>Kahn</last_name>
         </user>
     </users>
*/

--/
CREATE LUA SCALAR SCRIPT process_users(url VARCHAR(500))
EMITS (firstname VARCHAR(20), lastname VARCHAR(20)) AS
require("lxp")
HTTP = require("socket.http")
local in_user_tag = false;
local in_first_name_tag = false;
local in_last_name_tag = false;
local current = {}
local users = {}

p = lxp.new(
    {StartElement = function(p, tag, attr)
      if tag == "user" and attr.active == "1" then in_user_tag = true; end
      if tag == "first_name" then in_first_name_tag = true; end
      if tag == "last_name" then in_last_name_tag = true; end
    end,
    EndElement = function(p, tag)
      if tag == "user" then in_user_tag = false; end
      if tag == "first_name" then in_first_name_tag = false; end
      if tag == "last_name" then in_last_name_tag = false; end
    end,
    CharacterData = function(p, txt)
      if in_user_tag then
        if in_first_name_tag then current.first_name = txt; end
        if in_last_name_tag then current.last_name = txt; end
      end
      if current.first_name and current.last_name then
        users[#users+1] = current
        current = {}
      end
    end})
    
function run(ctx)
    content = http.request(ctx.url)
    p:parse(content); p:parse(); p:close();
    for i=1, #users do
      ctx.emit(users[i].first_name, users[i].last_name)
    end
end
/

SELECT process_users ('http://www.my_valid_webserver/goalies.xml')
FROM DUAL;