clustered index disadvantages in mysql

矫情吗；* 2021-12-23 02:13 153阅读 0赞

If you don’t define a primary key, InnoDB will try to use a unique nonnullable index  
instead. If there’s no such index, InnoDB will define a hidden primary key for you  
and then cluster on that.\* InnoDB clusters records together only within a page. Pages

with adjacent key values may be distant from each other.

These benefits can boost performance tremendously if you design your tables and queries  
to take advantage of them. However, clustered indexes also have disadvantages:  
• Clustering gives the largest improvement for I/O-bound workloads. If the data  
fits in memory the order in which it’s accessed doesn’t really matter, so clustering  
doesn’t give much benefit.  
• Insert speeds depend heavily on insertion order. Inserting rows in primary key  
order is the fastest way to load data into an InnoDB table. It may be a good idea  
to reorganize the table with OPTIMIZE TABLE after loading a lot of data if you  
didn’t load the rows in primary key order.  
• Updating the clustered index columns is expensive, because it forces InnoDB to  
move each updated row to a new location.  
• Tables built upon clustered indexes are subject to page splits when new rows are  
inserted, or when a row’s primary key is updated such that the row must be  
moved. A page split happens when a row’s key value dictates that the row must  
be placed into a page that is full of data. The storage engine must split the page  
into two to accommodate the row. Page splits can cause a table to use more  
space on disk.  
• Clustered tables can be slower for full table scans, especially if rows are less  
densely packed or stored nonsequentially because of page splits.  
• Secondary (nonclustered) indexes can be larger than you might expect, because  
their leaf nodes contain the primary key columns of the referenced rows.  
• Secondary index accesses require two index lookups instead of one.  
The last point can be a bit confusing. Why would a secondary index require two  
index lookups? The answer lies in the nature of the “row pointers” the secondary  
index stores. Remember, a leaf node doesn’t store a pointer to the referenced row’s  
physical location; rather, it stores the row’s primary key values.

转载于:https://www.cnblogs.com/perfectdesign/archive/2009/12/10/1621085.html