sql: Internal error when querying a REGIONAL BY ROW table during DROP REGION

[ajstorm]

If users query REGIONAL BY ROW tables while a background DROP REGION statement is occurring (more specifically, is in the schema changer), they may observe this:

ERROR: internal error: Optimize can only be called on relational root expressions
SQLSTATE: XX000
DETAIL: stack trace:
github.com/cockroachdb/cockroach/pkg/sql/opt/xform/optimizer.go:775: optimizeRootWithProps()
github.com/cockroachdb/cockroach/pkg/sql/opt/xform/optimizer.go:220: Optimize()
github.com/cockroachdb/cockroach/pkg/sql/plan_opt.go:525: buildExecMemo()
github.com/cockroachdb/cockroach/pkg/sql/plan_opt.go:194: makeOptimizerPlan()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:940: makeExecPlan()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:821: dispatchToExecutionEngine()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:659: execStmtInOpenState()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:122: execStmt()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:1527: func1()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:1529: execCmd()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:1450: run()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:484: ServeConn()
github.com/cockroachdb/cockroach/pkg/sql/pgwire/conn.go:626: func1()
runtime/asm_amd64.s:1374: goexit()

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The query then fails, but is likely to succeed if reissued.

This occurs because under the covers, the query detects the system in a partial state - the crdb_internal_region enum doesn't have the enum value present anymore, but the underlying RBR table hasn't yet been repartitioned.

The fix for this problem is to ensure that queries can't find the system in this partial state.