Exasol UDFs & Script Language Containers

Trigger when the user mentions UDF, user defined function, CREATE SCRIPT, ExaIterator, SCALAR, SET EMITS, BucketFS, script language container, SLC, exaslct, custom packages, GPU UDF, ctx.emit, ctx.next, variadic script, dynamic parameters, EMITS(...), default_output_columns, or any UDF/SLC-related topic.

When to Use UDFs

Use UDFs to extend SQL with custom logic that runs inside the Exasol cluster:

• Per-row transforms (cleaning, parsing, hashing)
• Custom aggregation across grouped rows
• ML model inference (load model from BucketFS, score rows)
• Calling external APIs from within SQL
• Batch processing with DataFrames

SCALAR vs SET Decision Guide

	SCALAR	SET
Input	One row at a time	Group of rows (via GROUP BY)
Output	RETURNS <type> (single value)	EMITS (col1 TYPE, ...) (zero or more rows)
Row iteration	Not needed	ctx.next() loop required
SQL usage	SELECT udf(col) FROM t	SELECT udf(col) FROM t GROUP BY key
Use case	Per-row transforms	Aggregation, ML batch predict, multi-row emit

Language Selection

Language	Startup	Best For	Expandable via SLC?
Python 3 (3.10 or 3.12)	~200ms	ML, data science, pandas, string processing	Yes
Java (11 or 17)	~1s	Enterprise libs, type safety, Virtual Schema adapters	Yes
Lua 5.4	<10ms	Low-latency transforms, row-level security	No (natively compiled into Exasol)
R (4.4)	~200ms	Statistical modeling, R model deployment	Yes

CREATE SCRIPT Syntax

Python SCALAR

CREATE OR REPLACE PYTHON3 SCALAR SCRIPT my_schema.clean_text(input VARCHAR(10000))
RETURNS VARCHAR(10000) AS
import re
def run(ctx):
    if ctx.input is None:
        return None
    return re.sub(r'[^\w\s]', '', ctx.input).strip().lower()
/

SELECT clean_text(description) FROM products;

Python SET

CREATE OR REPLACE PYTHON3 SET SCRIPT my_schema.top_n(
    item VARCHAR(200), score DOUBLE, n INT
)
EMITS (item VARCHAR(200), score DOUBLE) AS
def run(ctx):
    rows = []
    limit = ctx.n
    while True:
        rows.append((ctx.item, ctx.score))
        if not ctx.next():
            break
    rows.sort(key=lambda x: x[1], reverse=True)
    for item, score in rows[:limit]:
        ctx.emit(item, score)
/

SELECT top_n(product, revenue, 5) FROM sales GROUP BY category;

Java SCALAR

CREATE OR REPLACE JAVA SCALAR SCRIPT my_schema.hash_value(input VARCHAR(2000))
RETURNS VARCHAR(64) AS
import java.security.MessageDigest;

class HASH_VALUE {
    static String run(ExaMetadata exa, ExaIterator ctx) throws Exception {
        String input = ctx.getString("input");
        if (input == null) return null;
        MessageDigest md = MessageDigest.getInstance("SHA-256");
        byte[] hash = md.digest(input.getBytes("UTF-8"));
        StringBuilder hex = new StringBuilder();
        for (byte b : hash) hex.append(String.format("%02x", b));
        return hex.toString();
    }
}
/

Java with External JARs

CREATE OR REPLACE JAVA SCALAR SCRIPT my_schema.custom(input VARCHAR(2000))
RETURNS VARCHAR(2000) AS
  %scriptclass com.mycompany.MyProcessor;
  %jar /buckets/bfsdefault/default/jars/my-lib.jar;
/

Lua SCALAR

CREATE OR REPLACE LUA SCALAR SCRIPT my_schema.my_avg(a DOUBLE, b DOUBLE)
RETURNS DOUBLE AS
function run(ctx)
    if ctx.a == nil or ctx.b == nil then return null end
    return (ctx.a + ctx.b) / 2
end
/

R SET (ML Prediction)

CREATE OR REPLACE R SET SCRIPT my_schema.predict(
    feature1 DOUBLE, feature2 DOUBLE
)
EMITS (prediction DOUBLE) AS
run <- function(ctx) {
    model <- readRDS("/buckets/bfsdefault/default/models/model.rds")
    repeat {
        if (!ctx$next_row(1000)) break
        df <- data.frame(f1 = ctx$feature1, f2 = ctx$feature2)
        ctx$emit(predict(model, newdata = df))
    }
}
/

Variadic Scripts (Dynamic Parameters)

Use ... to accept any number of input columns, output columns, or both.

Dynamic Input

CREATE OR REPLACE PYTHON3 SCALAR SCRIPT schema.to_json(...) RETURNS VARCHAR(2000000) AS
import simplejson
def run(ctx):
    obj = {}
    for i in range(0, exa.meta.input_column_count, 2):
        obj[ctx[i]] = ctx[i+1]   # caller passes: name, value, name, value, ...
    return simplejson.dumps(obj)
/

SELECT to_json('fruit', fruit, 'price', price) FROM products;

• Access by index: ctx[i] — 0-based in Python/Java, 1-based in Lua/R
• Parameter names inside a variadic script are always 0, 1, 2, ... — never the original column names
• exa.meta.input_column_count — total number of input columns
• exa.meta.input_columns[i].name / .sql_type — per-column metadata

Dynamic Output (EMITS(...))

Declare EMITS(...) in CREATE SCRIPT. At call time, columns must be provided one of two ways:

Method	Where specified	Use when
EMITS in SELECT	Caller's SQL query	Output structure depends on data values
default_output_columns()	Script body	Output structure derivable from input column count/types alone

-- EMITS in SELECT (required when output depends on data content)
SELECT split_csv(line) EMITS (a VARCHAR(100), b VARCHAR(100), c VARCHAR(100)) FROM t;

# default_output_columns() — called before run(), no ctx/data access available
def default_output_columns():
    parts = []
    for i in range(exa.meta.input_column_count):
        parts.append("c" + exa.meta.input_columns[i].name + " " + exa.meta.input_columns[i].sql_type)
    return ",".join(parts)

If neither is provided, the query fails with:

The script has dynamic return arguments. Either specify the return arguments in the query via EMITS or implement the method default_output_columns in the UDF.

ExaIterator API Quick Reference

Python

Method/Property	SCALAR	SET	Description
ctx.<column>	yes	yes	Access input column value
return value	yes	no	Return single value (RETURNS)
ctx.emit(v1, v2, ...)	no	yes	Emit output row (EMITS)
ctx.emit(dataframe)	no	yes	Emit DataFrame as rows
ctx.next()	no	yes	Advance to next row; returns False at end
ctx.size()	no	yes	Number of rows in current group
ctx.reset()	no	yes	Reset iterator to first row
ctx.get_dataframe(num_rows, start_col)	no	yes	Get rows as pandas DataFrame

Important: There is no emit_dataframe() method — use ctx.emit(dataframe) to emit a DataFrame.

Java

Method	Description
ctx.getString("col")	Get string value
ctx.getInteger("col")	Get integer value
ctx.getDouble("col")	Get double value
ctx.getBigDecimal("col")	Get decimal value
ctx.getDate("col")	Get date value
ctx.getTimestamp("col")	Get timestamp value
ctx.next()	Advance to next row (SET only)
ctx.emit(v1, v2, ...)	Emit output row (SET only)
ctx.size()	Row count in group (SET only)
ctx.reset()	Reset to first row (SET only)

BucketFS File Access

All languages can read files from BucketFS at /buckets/<service>/<bucket>/<path>:

# Python — load a pickled ML model
import pickle
with open('/buckets/bfsdefault/default/models/model.pkl', 'rb') as f:
    model = pickle.load(f)

// Java — reference JARs via %jar directive
%jar /buckets/bfsdefault/default/jars/my-library.jar;

Performance tip: Load models/resources once (outside the row loop or in a module-level variable), not per-row.

GPU Acceleration (Exasol 2025.2+)

Exasol supports GPU-accelerated UDFs via CUDA-enabled Script Language Containers:

• Use template-Exasol-8-python-3.{10,12}-cuda-conda flavors
• Requires NVIDIA driver on the Exasol host
• Install GPU libraries (PyTorch, TensorFlow, RAPIDS) via conda in the SLC
• Standard UDF API — no code changes needed beyond importing GPU libraries

Script Language Containers (SLC) Overview

UDFs run inside Script Language Containers — Docker-based runtime environments. The default SLC includes standard libraries. When you need additional packages (e.g., scikit-learn, PyTorch, custom JARs), build a custom SLC.

When You Need a Custom SLC

• Installing pip/conda packages not in the default container
• Adding system libraries (apt packages)
• Using a different Python version (3.10 vs 3.12)
• Enabling GPU/CUDA support
• Adding R packages from CRAN

Quick Activation

-- Activate for current session
ALTER SESSION SET SCRIPT_LANGUAGES='PYTHON3=localzmq+protobuf:///<bfs-name>/<bucket>/<path>/<container>?lang=python#buckets/<bfs-name>/<bucket>/<path>/<container>/exaudf/exaudfclient_py3';

-- Activate system-wide (requires admin)
ALTER SYSTEM SET SCRIPT_LANGUAGES='...';

Install the Build Tool

pip install exasol-script-languages-container-tool

Performance Tips

• Load once, use many: Load models/resources outside the row loop
• Use SET for batching: Collect rows into a list/DataFrame, process in bulk
• Lua for low latency: Avoids JVM/Python startup overhead
• Parallelism is automatic: UDFs run on all cluster nodes simultaneously

Detailed References

• Python patterns — context API, DataFrame pattern, type mapping, testing: references/udf-python.md
• Java & Lua patterns — ExaMetadata API, JARs, adapters, Lua libraries: references/udf-java-lua.md
• Building custom SLCs — exaslct CLI, flavors, customization, deployment, troubleshooting: references/slc-reference.md