ClickHouse dialect: SHOW and EXPLAIN parse as Command (fallback for unknown syntax) #7548

Summary

In the clickhouse dialect, SHOW and EXPLAIN statements parse to exp.Command — sqlglot's catch-all for syntax it can't recognise — rather than to dedicated AST nodes. This makes them indistinguishable from genuinely-unknown statements like SYSTEM, RENAME, ATTACH, OPTIMIZE, etc., which also fall back to Command.

Reproduction (sqlglot 30.6.0)

import sqlglot

for s in ["SHOW TABLES",
          "SHOW CREATE TABLE t",
          "EXPLAIN SELECT 1",
          "EXPLAIN ESTIMATE SELECT 1"]:
    parsed = sqlglot.parse(s, dialect="clickhouse")
    print(s, "->", [type(p).__name__ for p in parsed])

Output:

'SHOW TABLES' contains unsupported syntax. Falling back to parsing as a 'Command'.
'SHOW CREATE TABLE t' contains unsupported syntax. Falling back to parsing as a 'Command'.
'EXPLAIN SELECT 1' contains unsupported syntax. Falling back to parsing as a 'Command'.
'EXPLAIN ESTIMATE SELECT 1' contains unsupported syntax. Falling back to parsing as a 'Command'.

SHOW TABLES -> ['Command']
SHOW CREATE TABLE t -> ['Command']
EXPLAIN SELECT 1 -> ['Command']
EXPLAIN ESTIMATE SELECT 1 -> ['Command']

For comparison, several other dialects do produce dedicated nodes for SHOW / EXPLAIN (e.g. exp.Show, exp.Explain).

Why it matters

In a downstream project we use sqlglot's ClickHouse dialect to validate that user-submitted SQL is read-only — accept SELECT / DESCRIBE, reject everything else. Because SHOW and EXPLAIN parse to the same Command node as SYSTEM / RENAME / ATTACH / OPTIMIZE, we can't safely allow them: accepting Command would also accept the write-capable statements. So we currently reject SHOW and EXPLAIN despite ClickHouse considering them read-only operations.

Folks doing access-control / SQL-firewall use cases with the ClickHouse dialect probably hit the same wall.

Suggested fix direction

Add ClickHouse-dialect parser support for SHOW and EXPLAIN so they produce exp.Show / exp.Explain (or dialect-specific subclasses) instead of Command. Both statements are well-defined in the ClickHouse docs — the grammar shouldn't be too painful, and dedicated nodes would unblock several validation/transformation use cases.

Happy to put up a PR if a maintainer can sketch the preferred shape.