After deleting all elements from an array, how much gas refund will be made?

Question

Following answer and solidity_link says that:

Use delete on arrays to delete all its elements.

delete dataArray;

This is exactly the same as dataArray.length = 0

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From this answer as I understand that negative gas costs will be 15000.

The only two OPCODEs with negative gas costs are STORAGEKILL(-15000) and GSUICIDEREFUND(-24000).

[Q] Overall, after deleting an array; how much gas refund will be made? Would it be only 15000 or 15.000 * 'length_of_the_array' ?

=> For example, I have an array with length of 100. When I delete it how much gas will I be refunded?

If only 15000; then should I traverse all the elements in an array and delete them rather than doing delete dataArray?

Something like:

for (uint i=0; i< dataArray.length; i++) { 
    delete dataArray[i];
}

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=> For example, if I want to delete an mapping that maps a struct (uint[] that has 100 items), would it refund 15000 or the gas cost of the struct that is pointed (15000 * 100).

If only 15000; then should I traverse all the elements mapped by the element as well and delete them rather than doing delete mappingData ?

mapping(address => uint[]) mappingData;  //'0xabcd' maps to length of the `uint[]` is 100
delete mappingData;

Something like:

for (uint i=0; i< mappingData[0xabcd].length; i++) {
    delete mappingData[0xabcd][i];
}

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Some observation: If I traverse an delete and delete items one by one, traversing will charge me additional gas cost, maybe equivalent to the refunded gas so where would be the motivation to use delete?

Answer 1

Opcode STORAGEKILL refund only applies to a single slot.

Deleting an array in Solidity is equivalent to deleting all slots used by the array. It should be 15000 * (1 + array_length * item_slots).

Here, item_slots is the number of storage slots used for each item in the array. For example, for an uint256 array, each item uses one slot. An array of struct with two uint256 will use two slots.

For more complex structs, you will have to check the Solidity documentation on how structs are lay out in storage to determine how many slots are used for each item.

For reference, each storage slot uses 32 bytes.

You should be aware the refund is done at the end of the transaction, you have to have enough gas to process each slot upfront. Also, your refund can be at most half of the gas used.