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If a table already exists in a smart contract on-chain and the table is full of entries, is it possible to make a secondary index for that table? Or would you have to create a new table and migrate all the data over to the new table?

Essentially, I guess I'm asking whether or not the secondary indices are calculated as required, or if they are also stored in RAM on chain.

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TL;DR

  • You need to migrate all rows.
  • Secondary indices are stored in RAM.

Each row of multi_index with secondary index is stored by multiple types of chainbase objects like key_value_object (for storing data by primary key) and indexTYPE_object (for storing primary key by secondary key).

This secondary index object (indexTYPE_object) only has primary key and can be found by secondary key. When you search for a row with secondary key, this secondary index object is found first, and eosio::multi_index makes you get an exact row by primary key which was stored in secondary index object.

This secondary index object is created or updated when you call eosio::multi_index::emplace/modify, so if you add secondary index to an existing multi_index table, existing rows don't have these secondary index objects, but new rows will have. This will break normal multi_index operations.

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