Everything I can find on the internet explains the difference between these types of indexing, that is NOT my question.
When I create a new table in a database, I always create an id primary key as I'm sure most people do. I always make it $timeout( function() {
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I'm recently getting big into adding custom indexes and I've noticed that my id field has these two index's (automatically created, for obvious reasons):
PRIMARY KEY, NOT NULL, UNIQUE, AUTO_INCREMENTMy assumption is due to the PK flag (primary key) the 'PRIMARY' index was automatically generated, and due to the There is a PRIMARY index
There is also a UNIQUE index
flag the 'UNIQUE' index was created.
I assume having two indexes on the same column is adding unnecessary overhead. My question is, is this true? Should I remove one of these index's? Or is it normal/ideal to have both of these index's created on my id column?
答案 0 :(得分：1)
PRIMARY KEY(x), UNIQUE(x) - 由于PRIMARY KEY 定义（在MySQL中）UNIQUE，因此后一索引是多余的，应该删除。
此外，在UNIQUE(x), INDEX(x)中，INDEX(x)是多余的。
冗余索引占用额外空间。 （在未来的版本中，它们甚至可能被禁止 - 这就是它们无用的。）
每张桌子应该（或者甚至＆＃34;必须＆＃34;）有PRIMARY KEY。它可以是AUTO_INCREMENT，也可以是某些＆＃34;自然＆＃34;键。 ＆＃34;天然＆＃34;从某种意义上说它是UNIQUE而永远不是NULL。在一项简短的调查中，我发现只有25％的表格有AUTO_INCREMENT列。
不要忘记考虑＆＃34;复合＆＃34;索引，例如INDEX(a,b)。