Add ClickHouse server-version detection

[JoshDreamland]

Exposes the ClickHouse server version to the planner via
chfdw_get_server_version(UserMapping*), with a chfdw_version_ge()
comparison helper. This is the reusable plumbing a follow-up will use to gate
subquery pushdown on the server version, so older ClickHouse releases degrade
gracefully instead of erroring on SQL their analyzer cannot handle.

What's here

• A server_version method on the libclickhouse_methods vtable, backed by a
  small dependency-free ch_server_version struct (shared between the
  Postgres-free driver internals and the FDW layer).
• Binary driver: reads the version straight from the native-protocol
  handshake (chc_client_server_info) — free once the connection exists.
• HTTP driver: ClickHouse exposes no version over HTTP (no response header,
  and the "connection" is just a curl handle with no server round-trip), so it
  fetches SELECT version() once and caches it on the connection.
• A clickhouse_server_version(text) SQL function returning
  major.minor.patch, which exercises both drivers and is covered by the new
  server_version regression test.

Notes

• No behavior change to existing flows: nothing calls the getter at plan time
  yet, so planning still opens no connection. The (amortized, once-per-session)
  plan-time connect arrives with the gate in the follow-up PR.

🤖 Generated with Claude Code

[Copilot]

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[serprex]

needs CHANGELOG entry

[serprex]

Suggested change

	/*
	* Build the query directly rather than via new_query(): this file does
	* not include fdw.h (which declares chfdw_get_session_settings), and the
	* version probe needs no session settings.
	*/

[JoshDreamland]

Deleted.

[theory]

Will read carefully tomorrow, but just to note here, if we're adding a new SQL function, we need to increment to v0.4.0 and add an upgrade script, sql/pg_clickhouse--0.3--0.4.sql. See #196 for a pervious such change to the extension's catalog.

[theory]

Simpler example of incrementing the version and adding an upgrade script in ClickHouse/pg_re2#22

[theory]

This looks good, though it doesn't look like the http connection caches the version info to allow multiple calls with the same connection without each resulting in an HTTP request.

Please add the new function to pg_clickhouse.md and an item to CHANGELOG.md, as well as the aforementioned migration script and version increments.

Also I thought ClickHouse returned a header with the version, but apparently not:

❯ curl -iX POST 'http://localhost:8123/' -d 'SELECT 1'
HTTP/1.1 200 OK
X-ClickHouse-Summary: {"read_rows":"1","read_bytes":"1","written_rows":"0","written_bytes":"0","total_rows_to_read":"1","result_rows":"0","result_bytes":"0","elapsed_ns":"5185292"}
Date: Wed, 24 Jun 2026 19:58:06 GMT
Connection: Keep-Alive
Content-Type: text/tab-separated-values; charset=UTF-8
Access-Control-Expose-Headers: X-ClickHouse-Query-Id,X-ClickHouse-Summary,X-ClickHouse-Server-Display-Name,X-ClickHouse-Format,X-ClickHouse-Timezone,X-ClickHouse-Exception-Code
X-ClickHouse-Server-Display-Name: clickhouse01
Transfer-Encoding: chunked
X-ClickHouse-Query-Id: 93bf9289-6dbf-4621-a5d9-9c183e8d8484
X-ClickHouse-Format: TabSeparated
X-ClickHouse-Timezone: UTC
Keep-Alive: timeout=3, max=9999

[theory]

Fuck the passive voice.

Suggested change

	The version is read from the connection — over the native protocol it comes
	from the handshake, and over the HTTP interface it is obtained with a single
	`SELECT version()` and cached for the life of the connection.
	Reads the version from the native protocol connection handshake or from a
	single `SELECT version()` http query and cached for the life of the
	connection.

[theory]

Suggested change

	* handshake, so this reads the cached handshake info rather than issuing a
	* handshake, so read the cached handshake info rather than issuing a

[theory]

Suggested change

	* to all out-params when the version cannot be determined. The result is cached
	* on the connection, so only the first call issues a query.
	* to all out-params when the version cannot be determined. Caches the result
	* on the connection, so only the first call issues a query.

[theory]

Suggested change

	* Added a `clickhouse_server_version(server)` function reporting the
	* Added the `clickhouse_server_version(server)` function, which reports the

[JoshDreamland]

Re:caching: this info lives on the connection itself (conn->version), yes, but specifically in the long-lived "gate" connection that all our scans use. So it's only re-fetched when a connection is re-made, which is the right behavior anyway or we could be connecting to a different version without knowing. You don't have to worry about it querying the version for each scan, though.

[JoshDreamland]

Deleted.

[JoshDreamland]

Comments updated; active voice, present indicative.

[theory]

Brilliant!