Is your feature request related to a problem?

Right now all codecs in custom-codecs repository delegates to PerFieldMappingPostingFormatCodec present in core OpenSearch. But core OpenSearch switches to CompositeCodec if index has mappings associated with composite index .

What solution would you like?

For indices where composite index is present, we should delegate to CompositeCodec instead of PerFieldMappingPostingFormatCodec.
I can think of couple of solutions :

1. Handle it based on mapperService.isCompositeIndexPresent() in custom-codecs repository
2. We can get the default delegate codec from core OpenSearch probably via CodecService based on MapperService inputs.