When the MySQL server receives a SELECT query, the query goes through several consecutive phases:
- parsing: SQL words are recognized, the query is split into different parts following the SQL grammar rules: a list of selected expressions, a list of tables to read, a WHERE condition, …
- resolution: the output of the parsing stage contains names of columns and names of tables. Resolution is about making sense out of this. For example, in “
WHERE foo=3“, “foo” is a column name without a table name; by applying SQL name resolution rules, we discover the table who contains “foo” (it can be complicated if subqueries or outer joins are involved).
- optimization: finding the best way to read tables: the best order of tables, and for each