Documentation: How to encrypt and decrypt existing tables.

[ImTheKai]

The following examples need to be covered within the documentation, as long as we don't have our own function for it

• How to encrypt an existing table
• How to decrypt and encrypted table (what's need to drop the encryption from the table)

Both is basically doable by creating a new table and syncing the data into the new table either by using the tde access method or heap.

[ImTheKai]

Will be tracked here https://perconadev.atlassian.net/browse/PG-826