问题
We have a multitenant application that has a table with 129 fields that can all be used in WHERE and ORDER BY clauses. I spent 5 days now trying to find out the best indexing strategy for us, I gained lot of knowledge but I still have some questions.
1) When creating an index should I always make it a composite index with tenant_id in the first place ?(all queries have tenant_id = ? in there WHERE clause)
2) Since all the columns can be used in both the WHERE clause and the order by clause, should I create an index on them all ? (right know when I order by a column that has no index it takes 6s to execute with a tenant that has about 1,500,000 rows )
3) make the PK (tenant_id, ID), but wouldn't this affect the joins to that table ?
Any advice on how to handle this would be much appreciated.
====== The database engine is InnoDB
=======
structure :
ID bigint(20) auto_increment primary
tenant_id int(11)
created_by int(11)
created_on Timestamp
updated_by int(11)
updated_on Timestamp
owner_id int(11)
first_name VARCHAR(60)
last_name VARCHAR(60)
.
.
.
(some 120 other columns that are all searchable)
回答1:
A few brief answers to the questions. As far as I can see you are confused with using indexes
Consider creating Indexes on columns if the Ratio -
Consideration 1 -
(Number of UNIQUE Entries of the Columns)/(Number of Total Entries in the Column) ~= 1
That is Count of DISTINCT rows in a particular column is high.
Creating an extra index will always create overhead for the MySQL server, so you MUST NOT create every column an index. There is also a limit on number of indexes your single table can have = 64 per table
Now if your tenant_id is present in all the search queries, you should consider it as an index or in a composite key,
provided that -
Consideration 2 - number of UPDATEs are less that number of SELECTs on the tenant_id
Consideration 3 - The indexes should be as small as possible in terms of data types. You MUST NOT create a varchar 64 an index
http://www.mysqlperformanceblog.com/2012/08/16/mysql-indexing-best-practices-webinar-questions-followup/
Point to Note 1 - Even if you do declare any column an index, MySQL optimizer may still not consider it as best plan of query execution. So always use EXPLAIN to know whats going on. http://www.mysqlperformanceblog.com/2009/09/12/3-ways-mysql-uses-indexes/
Point to Note 2 -
You may want to cache your search queries, so remember not to use unpredicted statements in your SELECT queries, such as NOW()
Lastly - making the PK (tenant_id, ID) should not affect the joins on your table.
And an awesome link to answer all your questions in general - http://www.percona.com/files/presentations/WEBINAR-MySQL-Indexing-Best-Practices.pdf
来源：https://stackoverflow.com/questions/14064489/mysql-composite-indexing-with-tenant-id