How to add Index to table column using Magento 2?

You can create your column as index key for better performance and speedy retrieval of data from a table.

An index is a performance optimization feature that enables data to be accessed faster compare to other nonindex columns. An index is used to more quickly find rows in the table based on the values which are part of the index and they don’t define the uniqueness of a column.

You can add your column as Index key using Magento 2 by below way using InstallSchema or UpgradeSchema.php file,

Here table_name is your table name of database and table_field is your field which you want to create index key.
You can check blog for Add foreign key to table, Add Foreign key to table using Magento 2

How to add Foreign Key in database table using Magento 2?

We can add a foreign key for a column of a table using Magento by the below code snippet.

Add foreign key in a column,
Example, you are creating a module and you want to add Store Filter in your module for that case you need to create a relation with store table.
Using below code snippet you can create a relationship with store table.

Here your custom table name is {my_table_name} and this table must contain field name store_id to a relation with store table.
In InstallSchema or UpgradeSchema File, you need to add below code snippet,

“my_table_name” is your table name
“store_id” is your table field name to relate with parent table
“store” Main table to create relation with
“store_id” Main table field name
“ACTION_CASCADE”  used for the type of actions which executes on a matched row of the child table when a parent table row gets update or delete action.

Using Above code You can add foreign key in your table which contain relation to store table of Magento.

You can refer blog for add Index to column by How to add Index to table column using Magento 2?

How to rename/change column name for database table using Magento 2?

In Magento 2 Sometimes we have added a column/field in the table for our custom requirement and in future if we need to rename or change that column name, we can rename it using UpgradeSchema.php file.

I have added one field name guestname in sales_order table and I want to rename that field to fullname. You can rename field using changeColumn() in UpgradeSchema.php file.
Using below code snippet you can rename already existed column field from a table.

Upgrade the setup_version number of module from module.xml file. An older version number is 1.0.1 so I have added new version 1.0.2 in module.xml file

Path: Magento22/app/code/Rbj/Training/etc/module.xml

Create UpgradeSchema.php file under Setup folder in your module.Path: Magento22/app/code/Rbj/Training/Setup/UpgradeSchema.php 

Using the above code snippet you can change column name from guestname to fullname with 255 character type text

Run command from root directory of your project,php bin/magento setup:upgrade

If you want to drop field name from a table, Remove an extra field from a table

If you want to add the field in table  Add New Field in Database Table Magento 2