1. Return result set size: The SELECT statement returns all rows of data that satisfy the query conditions, while the COUNT function only returns the number of rows that satisfy the conditions. As SELECT needs to return the complete result set to the caller, while COUNT only needs to return a numerical value, so when dealing with large amounts of data, the return result set size has a greater impact on performance.
2. Index utilization: COUNT function can usually be optimized using the index, especially when counting against a specific column, if the column has a suitable index, the database engine can directly use the index to count the number of rows that meet the conditions, avoiding a full table scan and improving the query speed.
3. The amount of data transmission: SELECT statement needs to send all the data that meets the conditions to the client, while the COUNT function only transmits a value, which reduces the amount of data transmitted over the network, and can accelerate the query speed to a certain extent.