Add duckdb_functions table function

[Mytherin]

This PR adds the duckdb_functions table function, that shows the list of functions currently built into the system, e.g.:

D select distinct on(function_name) function_name, function_type, return_type, parameters, parameter_types from duckdb_functions() where function_type='scalar' limit 10;
┌────────────────┬───────────────┬─────────────┬──────────────────────────┬──────────────────────────────────────┐
│ function_name  │ function_type │ return_type │        parameters        │           parameter_types            │
├────────────────┼───────────────┼─────────────┼──────────────────────────┼──────────────────────────────────────┤
│ log10          │ scalar        │ DOUBLE      │ [col0]                   │ [DOUBLE]                             │
│ mod            │ scalar        │ TINYINT     │ [col0, col1]             │ [TINYINT, TINYINT]                   │
│ date_diff      │ scalar        │ BIGINT      │ [col0, col1, col2]       │ [VARCHAR, DATE, DATE]                │
│ writefile      │ scalar        │ VARCHAR     │ []                       │ []                                   │
│ regexp_replace │ scalar        │ VARCHAR     │ [col0, col1, col2, col3] │ [VARCHAR, VARCHAR, VARCHAR, VARCHAR] │
│ age            │ scalar        │ INTERVAL    │ [col0]                   │ [TIMESTAMP]                          │
│ age            │ scalar        │ INTERVAL    │ [col0, col1]             │ [TIMESTAMP, TIMESTAMP]               │
│ datediff       │ scalar        │ BIGINT      │ [col0, col1, col2]       │ [VARCHAR, DATE, DATE]                │
│ map            │ scalar        │ MAP         │ []                       │ []                                   │
│ year           │ scalar        │ BIGINT      │ [col0]                   │ [TIMESTAMP WITH TIME ZONE]           │
└────────────────┴───────────────┴─────────────┴──────────────────────────┴──────────────────────────────────────┘

D select distinct on(function_type) * from duckdb_functions();
┌─────────────┬─────────────────────┬───────────────┬─────────────┬─────────────┬────────────────────┬─────────────────┬─────────┬────────────────────┐
│ schema_name │    function_name    │ function_type │ description │ return_type │     parameters     │ parameter_types │ varargs │  macro_definition  │
├─────────────┼─────────────────────┼───────────────┼─────────────┼─────────────┼────────────────────┼─────────────────┼─────────┼────────────────────┤
│ main        │ parquet_schema      │ table         │ NULL        │ NULL        │ [col0]             │ [VARCHAR]       │ NULL    │ NULL               │
│ main        │ log10               │ scalar        │ NULL        │ DOUBLE      │ [col0]             │ [DOUBLE]        │ NULL    │ NULL               │
│ main        │ count_star          │ aggregate     │ NULL        │ BIGINT      │ []                 │ []              │ NULL    │ NULL               │
│ main        │ table_info          │ pragma        │ NULL        │ NULL        │ [col0]             │ [VARCHAR]       │ NULL    │ NULL               │
│ pg_catalog  │ has_table_privilege │ macro         │ NULL        │ NULL        │ [table, privilege] │ [NULL, NULL]    │ NULL    │ CAST(t AS BOOLEAN) │
└─────────────┴─────────────────────┴───────────────┴─────────────┴─────────────┴────────────────────┴─────────────────┴─────────┴────────────────────┘

Currently the description and parameter names of functions are still missing. The idea is to add those to the system in a later refactor of the system.

The purpose of this function is to help with auto-generating of documentation (CC @Alex-Monahan) and to add support for displaying available functions of the system (e.g. using SHOW FUNCTIONS or similar).

[Alex-Monahan]

This will also be great for adding autocomplete to an IDE using DuckDB! Thanks Mark!

[mskyttner]

Could it be useful also for tab completion in the duckdb CLI? https://sqlite.org/completion.html mentions tab completion for keywords, and schema, table, and column names but mentions plans for adding also completions taken from function and pragma names.

And maybe for autocompletion in the duckdb_rest_server frontend? https://github.com/duckdb/duckdb/tree/master/tools/rest/frontend

[Mytherin]

Could it be useful also for tab completion in the duckdb CLI? https://sqlite.org/completion.html mentions tab completion for keywords, and schema, table, and column names but mentions plans for adding also completions taken from function and pragma names.

And maybe for autocompletion in the duckdb_rest_server frontend? https://github.com/duckdb/duckdb/tree/master/tools/rest/frontend

Yes, this would surely be useful for that.

[rsund]

COALESCE seems to be missing from the duckdb_functions() table. It is, however, listed in the duckdb_keywords() table.