What is the cost of SqL statement means?

Question

I have read references of cost of SQL statement everywhere in databases. What exactly does it mean? So this is the number of statements to be executed or something?

Answer 1

Cost is rough measure of how much CPU time and disk I/O server must spend to execute the query.

Typically cost is split into few components:

• Cost to parse the query - it can be non-trivial just to understand what you are asking for.
• Cost to get data from disk and access indexes if it reduces the cost.
• Cost to compute some mathematical operations on your data.
• Cost to group or sort data.
• Cost to deliver data back to client.

Answer 2

The cost is an arbitrary number that is higher if the CPU time/IO/Memory is higher. It is also vendor specific.