refactor: further clarify types of tables

[xxchan]

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What's changed and what's your intention?

follow up of #19510.
We've commented that TableCatalog contains different kinds of tables. In this PR we highlight user_tables (I think this name is not very cool, but cannot come up with a better one.)

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[xxchan]

• refactor: further clarify types of tables #19539 👈 (View in Graphite)
• main

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[xxchan]

This looks somehow wrong (but we prevented error somewhere else...)

dev=> create schema s;
dev=> create materialized view s.mv as select  1 as x;
dev=> drop schema s;
ERROR:  Failed to run the query

Caused by:
  Permission denied: PermissionDenied: The schema is not empty, try dropping them first


dev=> create table s.t (x int);
CREATE_TABLE
dev=> drop schema s;
ERROR:  Failed to run the query

Caused by these errors (recent errors listed first):
  1: Catalog error
  2: cannot drop schema s because table t depend on it

[xxchan]

Handled on meta here

risingwave/src/meta/src/controller/utils.rs

Lines 620 to 634 in 08fb7d4

	/// `ensure_schema_empty` ensures that the schema is empty, used by `DROP SCHEMA`.
	pub async fn ensure_schema_empty<C>(schema_id: SchemaId, db: &C) -> MetaResult<()>
	where
	C: ConnectionTrait,
	{
	let count = Object::find()
	.filter(object::Column::SchemaId.eq(Some(schema_id)))
	.count(db)
	.await?;
	if count != 0 {
	return Err(MetaError::permission_denied("schema is not empty"));
	}
	Ok(())
	}

[xxchan]

Can we simply remove the check on frontend? 🤔 cc @yezizp2012

[BugenZhao]

I think this name is not very cool, but cannot come up with a better one.

+1.

Shall we consider making it more type-safe, like, introducing wrappers such as UserTableCatalog or MaterializedViewCatalog and check the table_type field when creating them.

[BugenZhao]

Would you also help to review other methods within SchemaCatalog? For example, iter_all and iter_valid_table looks confusing.