try_cast_strict

This extension allows you to perform a strict conversion when casting a string to a target type. It can convert to any type DuckDB recognizes, yet it is particularly useful for converting to INTEGER and DECIMAL. In contrast to the DuckDB try_cast implementation, it does not round numbers, but produces a NULL value instead.

try_cast_strict takes two positional arguments:

• 1: the source to convert from, similar to try_cast. The source can be, for instance, the name of the column to convert from (e.g., my_column) or a string representation for a value (e.g., "1.2");
• 2: a string representation for the type to convert to (e.g., "INTEGER").

  NOTE: This is different from the try_cast implementation, where the name of the target type is a literal identifier as well.

Examples:

SELECT try_cast('1.12' AS DECIMAL(2,1)) AS result;
┌──────────────┐
│    result    │
│ decimal(2,1) │
├──────────────┤
│          1.1 │
└──────────────┘

SELECT try_cast_strict('1.12', 'DECIMAL(2,1)') AS result;
┌──────────────┐
│    result    │
│ decimal(2,1) │
├──────────────┤
│              │
└──────────────┘

SELECT try_cast_strict('1.1', 'DECIMAL(2,1)') AS result;
┌──────────────┐
│    result    │
│ decimal(2,1) │
├──────────────┤
│         1.1  │
└──────────────┘

SELECT try_cast('1.1' AS INTEGER) AS result;
┌────────┐
│ result │
│ int32  │
├────────┤
│      1 │
└────────┘

SELECT try_cast_strict('1.1', 'INTEGER') AS result;
┌────────┐
│ result │
│ int32  │
├────────┤
│        │
└────────┘

select try_cast_strict('1.11111119', 'FLOAT') as result;
┌───────────┐
│  result   │
│   float   │
├───────────┤
│ 1.1111112 │
└───────────┘

A three argument version is also available (try_cast_strict_sp), allowing to specify the decimal separator as the third argument.

Example:

SELECT try_cast_strict_sp('0,12e1', 'DECIMAL(3,1)', ',') AS result;
┌──────────────┐
│    result    │
│ decimal(3,1) │
├──────────────┤
│          1.2 │
└──────────────┘

Limitations

• Handles exponent notation only when casting to DECIMAL (i.e., not when casting to INTEGER).

Building

Managing dependencies

DuckDB extensions uses VCPKG for dependency management. Enabling VCPKG is very simple: follow the installation instructions or just run the following:

git clone https://github.com/Microsoft/vcpkg.git
./vcpkg/bootstrap-vcpkg.sh
export VCPKG_TOOLCHAIN_PATH=`pwd`/vcpkg/scripts/buildsystems/vcpkg.cmake

Note: VCPKG is only required for extensions that want to rely on it for dependency management. If you want to develop an extension without dependencies, or want to do your own dependency management, just skip this step. Note that the example extension uses VCPKG to build with a dependency for instructive purposes, so when skipping this step the build may not work without removing the dependency.

Build steps

Now to build the extension, run:

make

The main binaries that will be built are:

./build/release/duckdb
./build/release/test/unittest
./build/release/extension/try_cast_strict/try_cast_strict.duckdb_extension

• duckdb is the binary for the duckdb shell with the extension code automatically loaded.
• unittest is the test runner of duckdb. Again, the extension is already linked into the binary.
• try_cast_strict.duckdb_extension is the loadable binary as it would be distributed.

Running the extension

To run the extension code, simply start the shell with ./build/release/duckdb.

Now we can use the features from the extension directly in DuckDB. The template contains a single scalar function try_cast_strict() that takes a string arguments and returns a string:

D select try_cast_strict('1', 'INTEGER') as result;
┌───────────────┐
│    result     │
│    varchar    │
├───────────────┤
│             1 │ 
└───────────────┘

Running the tests

Different tests can be created for DuckDB extensions. The primary way of testing DuckDB extensions should be the SQL tests in ./test/sql. These SQL tests can be run using:

make test

Installing the deployed binaries

To install your extension binaries from S3, you will need to do two things. Firstly, DuckDB should be launched with the allow_unsigned_extensions option set to true. How to set this will depend on the client you're using. Some examples:

CLI:

duckdb -unsigned

Python:

con = duckdb.connect(':memory:', config={'allow_unsigned_extensions' : 'true'})

NodeJS:

db = new duckdb.Database(':memory:', {"allow_unsigned_extensions": "true"});

Secondly, you will need to set the repository endpoint in DuckDB to the HTTP url of your bucket + version of the extension you want to install. To do this run the following SQL query in DuckDB:

SET custom_extension_repository='bucket.s3.eu-west-1.amazonaws.com/<your_extension_name>/latest';

Note that the /latest path will allow you to install the latest extension version available for your current version of DuckDB. To specify a specific version, you can pass the version instead.

After running these steps, you can install and load your extension using the regular INSTALL/LOAD commands in DuckDB:

INSTALL try_cast_strict
LOAD try_cast_strict

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This repository is based on https://github.com/duckdb/extension-template, check it out if you want to build and ship your own DuckDB extension.