String Index

[brianjohnson5972]

This is a duplicate of #1623 (and there is no information here either)

[dominic-healid]

Any work around for this? need to add an index with a string type for the searching of texts.

[arhag]

Any work around for this? need to add an index with a string type for the searching of texts.

Don't expect native (i.e. intrinsic) support for a string index in EOSIO persistent storage anytime soon.

I strongly suggest you find an alternative design for your smart contract. For example, if you only care about lookup of exact string matches, then you can take a SHA256 hash of the string and store a 256-bit secondary index of that hash digest.

If you need to do actual lexicographical sorting of arbitrary length strings, then:

• First, re-evaluate whether you really need this as part of your smart contract or if it is something you can have in an off-chain DB that maps strings to primary keys, as part of the larger infrastructure supporting your application.
• If you decide you really do need this accessible to an on-chain smart contract then there may be ways you can implement it mostly on the WASM side by leveraging the existing database intrinsics. However, the implementation is quite involved and its performance and memory efficiency won't be as good as a natively supported string index. I won't get into the possible implementations here in this issue. Again, I strongly recommend figuring out if an alternative like the hash solution described above can work for your particular application.

[dominic-healid]

@arhag Thank you for your response.

I'm trying to follow your suggestion but I'm confuse to what exact data type should I use for this? You mentioned 256-bit, can you provide a sample?

[arhag]

@dominic-healid:

I don't know of a very good existing example specific to what you are trying to do.

But you can first take a look at how eosio::setabi action in the reference system contract handles taking a SHA256 hash of the ABI byte string and storing the digest in a table:

https://github.com/EOSIO/eosio.contracts/blob/v1.5.1/eosio.system/include/eosio.system/native.hpp#L71-L77

https://github.com/EOSIO/eosio.contracts/blob/v1.5.1/eosio.system/src/eosio.system.cpp#L269-L282

However, there are two important things to note about that example.

First, it is using the old capi_checksum256 C struct rather than the new eosio::checksum256 C++ type that was recently added in eosio.cdt. I recommend using the eosio::checksum256 type in the table rather than capi_checksum256, and you can generate the eosio::checksum256 from the SHA256 hash of arbitrary data (like your string) using the C++ eosio::sha256 function.

Second, it doesn't use the SHA256 digest as a 256-bit secondary key in the multi_index table, which you would need to do if you want to do lookups by the digest.

You can see an example of using a 256-bit secondary key in the old identity contract:

https://github.com/EOSIO/eos/blob/v1.4.4/contracts/identity/common.hpp#L23-L45

https://github.com/EOSIO/eos/blob/v1.4.4/contracts/identity/common.hpp#L64-L66

Except you would want to use the eosio::checksum256 rather than the now deprecated eosio::key256.