BaseShowTablesWithSizes: poor performance for MySQL 8.0 with thousands of tables

[shlomi-noach]

When the keyspace has thousands of tables, the tablet server engine takes a logs time to load. Measured numbers: about 10secfor4000` tables. The number can vary by machine type and tablet definitions.

The query is:

vitess/go/mysql/flavor_mysql.go

Lines 348 to 358 in d4c5096

	const TablesWithSize80 = `SELECT t.table_name,
	t.table_type,
	UNIX_TIMESTAMP(t.create_time),
	t.table_comment,
	SUM(i.file_size),
	SUM(i.allocated_size)
	FROM information_schema.tables t
	LEFT JOIN information_schema.innodb_tablespaces i
	ON i.name LIKE CONCAT(database(), '/%') AND (i.name = CONCAT(t.table_schema, '/', t.table_name) OR i.name LIKE CONCAT(t.table_schema, '/', t.table_name, '#p#%'))
	WHERE t.table_schema = database()
	GROUP BY t.table_name, t.table_type, t.create_time, t.table_comment`

The query can be optimized. Followup in a PR.

[shlomi-noach]

#13375 addresses this issue. Query time drops from 10s to some 0.2s or similar.