1

As per title, I'm using a key256 as a datatype for a secondary index on a multi-index table and want it to be unique. Can't find any documentation on it. TIA.

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Yes you can, but it is optional.

From the docs:

the primary index type must be uint64_t and must be unique, but the other, secondary, indexes can have duplicates


Example also indicating that secondary index may be unique:

struct [[eosio::table]] poll 
{
  uint64_t key; // primary key
  uint64_t pollId; // second key, can be non-unique
  std::string pollName; // name of poll
  uint8_t pollStatus =0; // staus where 0 = closed, 1 = open, 2 = finished
  std::string option; // the item you can vote for
  uint32_t count =0; // the number of votes for each time

  uint64_t primary_key() const { return key; }
  uint64_t by_pollId() const {return pollId; }
};

The standard way of making this unique is in your creation of the row by adding the available_primary_key() method:

Returns An available (unused) primary key value.

_polls.emplace(get_self(), [&](auto& p) {
        p.key = _polls.available_primary_key(); // method
        p.pollId = _polls.available_primary_key();
        p.pollName = pollName;
        p.pollStatus = 0;
        p.option = "";
        p.count = 0;
    });

Link to method in docs: https://developers.eos.io/eosio-cpp/reference#available_primary_key

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