举例说明，假如数据库管理人员想要知道某个MySQL数据库实例中存储空间的概况，那么能够通过执行如下的对数据字典的查询来实现：
mysql> SELECT a.schema_name db_name, -> IFNULL(ROUND((SUM(b.data_length) SUM(b.index_length))/1024/1024,2),0.00) -> total_size_mb, -> IFNULL(ROUND(((SUM(b.data_length) SUM(b.index_length))- -> SUM(b.data_free))/1024/1024,2),0.00) data_used_mb, -> IFNULL(ROUND(SUM(data_free)/1024/1024,2),0.00) data_free_mb, -> IFNULL(ROUND((((SUM(b.data_length) SUM(b.index_length))-SUM(b.data_free))/ -> ((SUM(b.data_length) SUM(b.index_length)))*100),2),0) pct_used, -> COUNT(table_name) tables -> FROM information_schema.schemata a -> LEFT JOIN information_schema.tables b ON a.schema_name = b.table_schema -> WHERE a.schema_name != 'information_schema' -> GROUP BY a.schema_name -> ORDER BY 1; --------- --------------- -------------- -------------- ---------- -------- | db_name | total_size_mb | data_used_mb | data_free_mb | pct_used | tables | --------- --------------- -------------- -------------- ---------- -------- | gim | 432.67 | 432.67 | 0.00 | 100.00 | 16 | | gim2 | 8.64 | 8.64 | 0.00 | 100.00 | 6 | | mysql | 0.33 | 0.33 | 0.00 | 99.69 | 18 | | test | 0.00 | 0.00 | 0.00 | 0.00 | 0 | | tpcc | 126.09 | 126.09 | 0.00 | 100.00 | 9 | --------- --------------- -------------- -------------- ---------- --------注意上面的示例中，应该设为不查询字典数据库自身，通过where条件语句来实现。
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