fix(splitter): do not split EXPLAIN from the explained statement

[amogiska]

What kind of change does this PR introduce?

Bug fix.

What is the current behavior?

The statement splitter treats the statement after an EXPLAIN prefix as a new statement, so EXPLAIN SELECT 1 is split into two statements (EXPLAIN + SELECT 1):

splitStatements('EXPLAIN SELECT 1')
// => [{ sql: 'EXPLAIN' }, { sql: 'SELECT 1' }]

Downstream, lint() then reports valid SQL as invalid — the bare EXPLAIN fragment fails to parse with Invalid statement: syntax error at end of input — and consumers that execute the split statements send a broken bare EXPLAIN to Postgres. All EXPLAIN forms are affected (EXPLAIN ANALYZE, EXPLAIN (ANALYZE, BUFFERS), etc.).

This is the same mis-split class previously fixed for UNION (#321) and GRANT (#417).

What is the new behavior?

EXPLAIN is handled as a statement prefix: a dedicated explain() splitter function consumes the prefix — the parenthesized option list or the legacy [ANALYZE | ANALYSE] [VERBOSE] modifiers — and then delegates to the splitter function of the explained statement, mirroring how statement() dispatches. Covered forms include EXPLAIN SELECT/INSERT/UPDATE/DELETE, EXPLAIN WITH ... SELECT, EXPLAIN CREATE TABLE AS, EXPLAIN VALUES, and multi-statement sources (EXPLAIN SELECT 1; SELECT 2 splits at the semicolon).

EXPLAIN_KW is deliberately not added to STATEMENT_START_TOKENS: explain is an unreserved keyword and remains valid as an identifier mid-statement (e.g. SELECT explain FROM t), which a break-on-EXPLAIN rule in unknown() would mis-split. A regression test covers this.

Additional context

Found while integrating @postgres-language-server/wasm for SQL editing/execution at ClickHouse, where this surfaced both as lint false positives and as broken EXPLAIN execution.

[psteinroe]

thanks for the contribution! can you share more about your use case at clickhouse? happy to make the language server work for you!

[lneves12]

@psteinroe hey! Clickhouse released recently a managed Postgres offering. We are using it inside our SQL Editor. Thank you for the great work so far!