SQL: Index and Optimization

Overview

To be honest, I'm not a pro-SQL programmer. I'm still on my journey to learn more about database and query optimization. In this blog I will just give whatever I've learnt about indexing and optimization and its mostly based on MySQL. Hope it helps!

Guidelines

1. Single Sheet query is much better than Multiple Sheet
2. If multiple sheet is needed, Use JOIN well:
   • Small Sheet drive Large Sheet (for e.g. left join in this case)
   • Establish proper indexing
   • Don't JOIN too many sheets as well
3. Try best NOT to use subquery or Cartesian Product
4. Window Funtions can be very helpful

Indexes

• Allow faster retrieval of data
• Question: Why don't we just create loads of indexes?
• Ansewr: There is a trade-off, if loads of indexes exists on a table then those indexes need to be updated or maintained. In this case, DML operations suffer.

1. Index operations

-- show indices
SHOW INDEX FROM your_db_name.customer;

-- Add index
ALTER TABLE payment
ADD INDEX idx_pay (payment_id);  -- [index] can be appended by [unique] to ensure each index is unique

CREATE FULLTEXT INDEX idx_staff ON customer (email); -- [fulltext] only applicable to string data

-- Drop Index
DROP INDEX idx_pay ON payment

For the full list of operations, you may refer to the official documentation of MySQL¹

2. Clustered Indexes

• ALTER TABLE Permission
• WHen a table does not have a clustered index then the table is stored as a heap, if the table has a clustered index it is stored as a B-tree
• Data is stored in order of clustered index
• Only one clustered index can exists on one table
• Clustered indexes are effective on columns that consistent of unique increasing integers (like identity_set)
• When a primary key is created a unique clustered index is automatically created - this can be beneficial for queries that involve joins on this column.

TODO

• Discuss B-Tree
• Study B+Tree and update