The parser stage consists of two parts:
The parser defined in
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
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.
scan.l is transformed
to the C source file
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
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
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
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
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
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