show index from tb_name
關於MySQL的show index from tb_name 的各列說明
table : 表名
non_unique：如果索引不能包括重復詞，則為0。如果可以，則為1。
key_name：索引名字
seq_in_index：索引序列號，從1開始
column_name：列名稱
collation：存儲方式，A為升序，null無分類
cardinality：索引中唯一數目的估計值。這個基數值越大，在做表連接時，選擇這個索引的機會就越大。
sub_part：如果列只是被部分地編入索引，則為被編入索引的字符的數目。如果整列被編入索引，則為NULL。
packed：指示關鍵字如何被壓縮。如果沒有被壓縮，則為NULL。
null：如果列含有NULL，則含有YES。如果沒有，則該列含有NO。
index_type：用的索引方法（BTREE, FULLTEXT, HASH, RTREE）。
comment：評註
index_comment：索引評註
visible：是否可見，隱藏？
（有個SQL，開發讓加一個索引，然後很明顯，執行時並沒有走這個索引。應該這個索引並不是最優索引）
show index from tb_name
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