How to loop a multi_index table in a smart contract

Question

I have very simple table items with the following structure:

struct [[eosio::table]] item
   {
       uint64_t id;
       uint64_t state;
       string description;

       uint64_t primary_key() const { return id; }
       uint64_t by_state() const { return state; }
   };

   typedef eosio::multi_index<"items"_n, item,
       eosio::indexed_by<"bystate"_n, eosio::const_mem_fun<item, uint64_t, &item::by_state>>> items;
    items items_table;

and I have an action within smart contract, where I need to filter the items by particular state and loop the result executing a simple processing for each of the filtered items. Currently I am using the following implementation (simply looping the whole table, searching for the items on specified state):

for (auto iter = items_table.begin(); iter != items_table.end(); iter++)
{
   if (iter->state == state)
   {
       // handle item record
       ...
   }

}

but it is not sufficient if table contains enormous amount of items.

Does anyone knows is there a way (is EOS) to fetch the item records on specified state using secondary index "bystate" and further loop those records and apply another logic for each of them ?

Answer 1

With your current setup, you could do something like this to optimize things a bit:

items_table items(_self, _self.value);

auto idx = items.get_index<"bystate"_n>();
auto itr_start = idx.lower_bound(name("pending").value);
auto itr_end = idx.upper_bound(name("pending").value);

for (itr_start != itr_end; itr_start++)
{
       // handle item record (they should all be of state "pending")
}

However, you still have to be aware of the transaction time limit as mentioned above.

Answer 2

This is a very common question, because the transaction time limit is going to kick in. At the moment I am using deferred transactions to trigger a search action that takes an additional argument.

You could also save the state of your search in another table as well.

I discussed this in a thread with the deletion of elements, you can adapt the code to use it to search.

Delete all multi_index records without iterator?