The parser stage consists of two parts:
       The parser defined in
       gram.y and scan.l is
       built using the Unix tools bison
       and flex.
      
The transformation process does modifications and augmentations to the data structures returned by the parser.
The parser has to check the query string (which arrives as plain text) for valid syntax. If the syntax is correct a parse tree is built up and handed back; otherwise an error is returned. The parser and lexer are implemented using the well-known Unix tools bison and flex.
     The lexer is defined in the file
     scan.l and is responsible
     for recognizing identifiers,
     the SQL key words etc. For
     every key word or identifier that is found, a token
     is generated and handed to the parser.
    
     The parser is defined in the file gram.y and
     consists of a set of grammar rules and
     actions that are executed whenever a rule
     is fired. The code of the actions (which is actually C code) is
     used to build up the parse tree.
    
     The file scan.l is transformed to the C
     source file scan.c using the program
     flex and gram.y is
     transformed to gram.c using
     bison.  After these transformations
     have taken place a normal C compiler can be used to create the
     parser. Never make any changes to the generated C files as they
     will be overwritten the next time flex
     or bison is called.
     
The mentioned transformations and compilations are normally done automatically using the makefiles shipped with the PostgreSQL source distribution.
     A detailed description of bison or
     the grammar rules given in gram.y would be
     beyond the scope of this paper. There are many books and
     documents dealing with flex and
     bison. You should be familiar with
     bison before you start to study the
     grammar given in gram.y otherwise you won't
     understand what happens there.
    
The parser stage creates a parse tree using only fixed rules about the syntactic structure of SQL. It does not make any lookups in the system catalogs, so there is no possibility to understand the detailed semantics of the requested operations. After the parser completes, the transformation process takes the tree handed back by the parser as input and does the semantic interpretation needed to understand which tables, functions, and operators are referenced by the query. The data structure that is built to represent this information is called the query tree.
     The reason for separating raw parsing from semantic analysis is that
     system catalog lookups can only be done within a transaction, and we
     do not wish to start a transaction immediately upon receiving a query
     string.  The raw parsing stage is sufficient to identify the transaction
     control commands (BEGIN, ROLLBACK, etc), and
     these can then be correctly executed without any further analysis.
     Once we know that we are dealing with an actual query (such as
     SELECT or UPDATE), it is okay to
     start a transaction if we're not already in one.  Only then can the
     transformation process be invoked.
    
     The query tree created by the transformation process is structurally
     similar to the raw parse tree in most places, but it has many differences
     in detail.  For example, a FuncCall node in the
     parse tree represents something that looks syntactically like a function
     call.  This might be transformed to either a FuncExpr
     or Aggref node depending on whether the referenced
     name turns out to be an ordinary function or an aggregate function.
     Also, information about the actual data types of columns and expression
     results is added to the query tree.