Description

A command-line, local SQL engine.

Inspiration:

• Powershell that can pipe structured objects
• The amazing LogParser that allows querying log files with sql

If you are looking something more serious, look into nushell (with relies on polars) and duckdb, they are amazing.

This is only a coding experiment to pursue several goals:

• Play with Scala 3
• Play with Shapeless 3
• Play with Recursion schemes
• Build a rudimentary SQL engine, from scratch, because why not

Example

It is intended to run queries like:

SELECT
    regex_struct(
        /(?<user>[^ ]+)\s+(?<pid>[^ ]+)\s+(?<command>[^ ]+)/,
        col_0) as user
FROM RUN('ps axo user:20,pid,comm') as ps

and

SELECT
    regex_struct(
        /(?:[^ ]+\s+){2}(?<user>[^ ]+)\s+(?:[^ ]+\s+)(?<size>[^ ]+)\s+(?:[^ ]+\s+){3}(?<filename>[^ ]+)/,
        col_0) as user
FROM RUN('ls -la') as ls

but also really useful queries like

SELECT
  1 AS one
  , 1 * 42 AS three
  ,a.b + 1 AS four
  ,a.d -1 AS five
  ,a.d -1 + 1 AS six
  ,max(a.c - 1) + 1 AS seven
FROM
  (SELECT 1 AS b, 1 AS d, 1 AS c) AS a
GROUP BY a.b, a.d - 1

Structure

• A minimal sql parser with very precise error reporting
• A minimal sql analyzer with types with very precise error reporting
• Eval based on catamorphism
• A minimal runner and planner
• Token-level error reporting

ERROR: Expressions in GROUP BY clause should not contain aggregation functions

SELECT 1 FROM (SELECT 1 AS bar, 2 AS baz) AS foo GROUP BY foo.bar, array(max(foo.bar))
                                                                         ^^^^^^^^^^^^

Interesting

• Replayable Fs2 topic allowing late subscriptions
• A queue allowing to dequeue with predicates
• Populate (and typecheck) objects based on regex groups naming

Esoteric

• FunctorK implementation with scala3 derivation