What does parsing a query mean?

Question

Most relational databases handles a JDBC / SQL query in four steps:

1. Parse the incoming SQL query
2. Compile the SQL query
3. Plan/optimize the data acquisition path
4. Execute the optimized query / acquire and return data

I want to know what does "parse the incoming query" really mean? And what does "plan/optimize data acquisition path" mean?

Answer 1

1. Parsing means examining the characters input and recognizing it as a command or statement by looking through the characters for keywords and identifiers, ignoring comments, arranging quoted portions as string constants, and matching the overall structure to the language syntax making sense of it all.

2. Plan/optimize means figure out the best way (of all the possible ways) to determine the result, usually with respect to execution time. It could also mean minimizing the number of locks needed. Maybe some parts of the query can be ignored (where ... and 1 == 1) or a table doesn't need to be accessed at all, etc.