Where should tables be created? In the contract or in each action?

Question

In the simple blog dapp (https://github.com/EOSIO/eosio-project-demux-example) the table (for posts) was defined in the contract but is not instantiated as a field.

What are the advantages/disadvantages using a contract field over creating the table in each action. Is one more efficient RAM/CPU/bandwith-wise for the user or the contract account?

Code for creating in each action: void action(...) { post_table poststable(_self, _self); posttable.find(...); } ...

Code for field of contract:

explicit blog(account_name self) : contract(self) poststable(_self, _self) {} post_table poststable; void action(...) { posttable.find(...); ...

Answer 1

Both options are valid and it depends on the case. The first argument of multi_index object is the "owner" (code) of the table and the second is the "context" (scope). In practice you have a different set of table rows for each scope.

Answering your question, if you create the multi_index object inside an action, you may use the parameters to determine the table scope and therefore its content. That initialization make the action to consume more CPU than if the multi_index object were initialized in the constructor once.

On the other hand,creating a multi_index object in the constructor is optimal from CPU perspective but you can't change the scope of that table.

In your example, creating a multi_index object with parameters (_self, _self) is better to do it in the constructor.

Answer 2

Actually this code is to execute table actions which is definitely defined in hpp file as structure.In execution of emplace action on table it uses ram memory and after execution of modify action on table it uses cpu cycles and net bandwidth.