Lua support for Virtual Schema Adapter Scripts
Details
| Detail name | Value |
|---|---|
| Changelog Number | 10671 |
| Type | New Feature |
| Status | Resolved |
| Fix Versions | Exasol 7.1.0, Exasol 7.1.rc1 |
| Resolution Date | 2021-08-03 |
Background
Virtual schemas allow virtually accessing external data sources. Virtual Schemas require Adapter Scripts, that can be implemented in Java and Python.
New Feature
Adapter scripts can now also be written in Lua. Such adapters have a much smaller runtime overhead and can execute local SELECT queries transaction-safe.
New SQL Syntax
CREATE [OR REPLACE] LUA ADAPTER SCRIPT script_name AS
script_content
/
New Lua API
Callback Function
The virtual schema callback function has the following signature in Lua:
function adapter_call(request_json)
The Lua CJSON library can be used for JSON handling, see EXASOL-2598.
Database Interaction
During the execution of the statements CREATE VIRTUAL SCHEMA, ALTER VIRTUAL SCHEMA, and DROP VIRTUAL SCHEMA (i.e., during the Virtual Schema API request types "createVirtualSchema", "refresh", "setProperties", and "dropVirtualSchema") the Lua adapter script can execute SQL statements. The statements run locally in the same transaction and session as the virtual schema statements. The API for this is similar to the query and pquery functions, but instead there are two new functions which are only available in Lua adapter scripts: exa.query_no_preprocessing and exa.pquery_no_preprocessing.
The following restrictions are enforced for these functions in Lua adapter scripts:
- The queries always run with the privileges of the virtual schema owner.
- No preprocessor script is used for the queries. This makes the queries safe from modification by the user of the virtual schema.
- Only SELECT queries are allowed.
- Not allowed in batch mode.
- No query caching.
