Hi @sql365 ,
Always On group
You can add indexes online with SQL Server Enterprise Edition in SQL AG as shoebshabibi-3140 mentioned. Index created on AG primary replica will be updated on secondary replica automatically.
Indexes can be added at the Publisher or Subscribers with no special considerations for replication (be aware that indexes can affect performance). CREATE INDEX and ALTER INDEX are not replicated, so if you add or change an index at, for example, the Publisher, you must make the same addition or change at the Subscriber if you want it reflected there.
Refer to MS document.
EVERYTHING is mirrored include index. Index created in principal server will be mirrored to mirror server.
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