storage/sources/postgres: avoid running incompatible queries against PostgreSQL <15

[benesch]

When fetching schema info from PostgreSQL, the PostgreSQL source currently issues a query that's known to be compatible only with v15+. If the query fails, the source retries with a version of the query that's compatible with older versions of PostgreSQL:

materialize/src/postgres-util/src/schemas.rs

Lines 114 to 158 in 164f8b4

	// PG 15 adds UNIQUE NULLS NOT DISTINCT, which would let us use `UNIQUE` constraints over
	// nullable columns as keys; i.e. aligns a PG index's NULL handling with an arrangement's
	// keys. For more info, see https://www.postgresql.org/about/featurematrix/detail/392/
	let pg_15_plus_keys = "
	SELECT
	pg_constraint.oid,
	pg_constraint.conkey,
	pg_constraint.conname,
	pg_constraint.contype = 'p' AS is_primary,
	pg_index.indnullsnotdistinct AS nulls_not_distinct
	FROM
	pg_constraint
	JOIN
	pg_index
	ON pg_index.indexrelid = pg_constraint.conindid
	WHERE
	pg_constraint.conrelid = $1
	AND
	pg_constraint.contype =ANY (ARRAY['p', 'u']);";
	// As above but for versions of PG without indnullsnotdistinct.
	let pg_14_minus_keys = "
	SELECT
	pg_constraint.oid,
	pg_constraint.conkey,
	pg_constraint.conname,
	pg_constraint.contype = 'p' AS is_primary,
	false AS nulls_not_distinct
	FROM pg_constraint
	WHERE
	pg_constraint.conrelid = $1
	AND
	pg_constraint.contype =ANY (ARRAY['p', 'u']);";
	let keys = match client.query(pg_15_plus_keys, &[&oid]).await {
	Ok(keys) => keys,
	Err(e)
	// PG versions prior to 15 do not contain this column.
	if e.to_string()
	== "db error: ERROR: column pg_index.indnullsnotdistinct does not exist" =>
	{
	client.query(pg_14_minus_keys, &[&oid]).await.map_err(PostgresError::from)?
	}
	e => e.map_err(PostgresError::from)?,
	};

While this works fine for customers running versions of PostgreSQL before v15, it has an annoying side effect: those customers can observe the failed queries in their PostgreSQL log files. This has led at least two customers to raise concerns:

• https://materializeinc.slack.com/archives/C04AW6GF29H/p1726498193951619
• https://materializeinc.slack.com/archives/C06KYARK95Z/p1714073989765699?thread_ts=1714070437.863809&cid=C06KYARK95Z

We should consider switching to sniffing out the version and deciding up front which query to run, to avoid running a query that's guaranteed to fail against older versions of PostgreSQL.