What is the difference between Magento 2.2 and 2.3?
Are Magento 2.3 DB schema and coding structure same as compare to Magento 2.2?
Are Magento 2.2 extension supported with Magento 2.3?
Can we customize own extension in Magento 2.3?
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Feature wise difference between Magento 2.2 and Magento 2.3
Declarative DB Schema
Magento 2.3 introduces a new approach – declarative database schema, allowing developers to define database structures and changes in XML files, instead of defining with install scripts. Magento 2.3 will convert the XML schema files into SQL statements and perform them.
The new declarative schema provides a big advantage: rollback support. This means you can roll back database changes you implement on a module or functionality to a previous version
And yes you can customize your own extension
I hope this will help