The process of parsing SQL queries involves converting plain text statements into a parse tree, which can be further processed by other subsystems.
An example query, such as the one shown below, can be analyzed in this manner:
testdb=# SELECT id, data FROM tbl_a WHERE id < 300 ORDER BY data;
The root node of the parse tree is the SelectStmt structure defined in parsenodes.h. Each element of the SELECT query corresponds to a specific element in the parse tree, identified by the same numbering. For instance, (1) represents an item in the target list, specifically the 'id' column of the table, while (4) indicates a WHERE clause.
It is important to note that the parser solely focuses on checking the syntax of the input query. If there is a syntax error, an error will be returned. However, semantic checks, such as verifying the existence of a table, are not performed by the parser. These checks are carried out by the analyzer/analyser, which goes beyond syntax and evaluates the meaning and validity of the query.
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