fix: refuse a CREATE on a name a table already holds

[robinskil]

Problem

Two defects, both about the name a table holds.

CREATE EXTERNAL TABLE and CREATE VIEW registered over whatever held the name. A typo repointed a table or swapped a view with no warning. CREATE TABLE and CREATE MATERIALIZED VIEW refused the same name, so one typo destroyed a table and another failed, by statement. The admin API documented the behavior it did not have: the if_not_exists field of POST /api/admin/external-tables skips "instead of erroring", and nothing errored.

Beacon also registers an empty stand-in table at startup, so a JSON query without a from field reports no missing table. The stand-in ignored sql.default_table and always took the literal name default. A server with BEACON_DEFAULT_TABLE=observations therefore held a table called default and left observations missing.

Change

Every CREATE refuses a name that a table or a view holds. The modifiers the SQL reference already documents do the work:

Statement	On a taken name
CREATE [EXTERNAL] TABLE	Error
CREATE VIEW, CREATE MATERIALIZED VIEW	Error
CREATE EXTERNAL TABLE IF NOT EXISTS	Keeps the table, reports success
CREATE OR REPLACE EXTERNAL TABLE, CREATE OR REPLACE VIEW	Overwrites

The paths that replace a provider on purpose register directly and do not change: a materialized-view REFRESH, an ALTER TABLE, and a crawler that re-registers a table it owns.

Startup registers the stand-in under the configured name, and only when the name is free. Drop it to take the name:

DROP TABLE "default";
CREATE EXTERNAL TABLE "default" STORED AS PARQUET LOCATION 'obs/';

That table then survives a restart. The error on the stand-in names it and tells you to drop it, because a table nobody made is a confusing thing to collide with.

Breaking

A script that repoints a table with a bare re-CREATE needs OR REPLACE, or a DROP TABLE in front.

Tests

7 tests in tests/default_table.rs, 5 in tests/external_tables.rs, 3 unit tests, 1 HTTP test.

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.44%. Comparing base (b27e545) to head (7bf8f96).

Files with missing lines	Patch %	Lines
...eacon-core/src/schema_persistence/default_table.rs	88.57%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #479      +/-   ##
==========================================
+ Coverage   83.41%   83.44%   +0.02%     
==========================================
  Files         374      375       +1     
  Lines       63237    63334      +97     
==========================================
+ Hits        52749    52848      +99     
+ Misses      10488    10486       -2     

Files with missing lines	Coverage Δ
...eacon-db/beacon-core/src/schema_persistence/mod.rs	92.85% <100.00%> (+0.26%)	⬆️
...-db/beacon-core/src/schema_persistence/provider.rs	96.78% <100.00%> (+0.43%)	⬆️
...eacon-db/beacon-core/src/statement_plan/actions.rs	80.32% <100.00%> (+1.06%)	⬆️
...eacon-core/src/statement_plan/materialized_view.rs	92.30% <100.00%> (+1.00%)	⬆️
...acon-db/beacon-core/src/statement_plan/physical.rs	85.68% <100.00%> (+0.06%)	⬆️
...db/beacon-core/src/statement_plan/query_planner.rs	84.21% <ø> (ø)
...eacon-core/src/schema_persistence/default_table.rs	88.57% <88.57%> (ø)

... and 3 files with indirect coverage changes

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