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c9d5298485
Invent new RawParseModes that allow the core grammar to handle
pl/pgsql expressions and assignments directly, and thereby get rid
of a lot of hackery in pl/pgsql's parser. This moves a good deal
of knowledge about pl/pgsql into the core code: notably, we have to
invent a CoercionContext that matches pl/pgsql's (rather dubious)
historical behavior for assignment coercions. That's getting away
from the original idea of pl/pgsql as an arm's-length extension of
the core, but really we crossed that bridge a long time ago.
The main advantage of doing this is that we can now use the core
parser to generate FieldStore and/or SubscriptingRef nodes to handle
assignments to pl/pgsql variables that are records or arrays. That
fixes a number of cases that had never been implemented in pl/pgsql
assignment, such as nested records and array slicing, and it allows
pl/pgsql assignment to support the datatype-specific subscripting
behaviors introduced in commit c7aba7c14.
There are cosmetic benefits too: when a syntax error occurs in a
pl/pgsql expression, the error report no longer includes the confusing
"SELECT" keyword that used to get prefixed to the expression text.
Also, there seem to be some small speed gains.
Discussion: https://postgr.es/m/4165684.1607707277@sss.pgh.pa.us
src/backend/nodes/README
Node Structures
===============
Andrew Yu (11/94)
Introduction
------------
The current node structures are plain old C structures. "Inheritance" is
achieved by convention. No additional functions will be generated. Functions
that manipulate node structures reside in this directory.
FILES IN THIS DIRECTORY (src/backend/nodes/)
General-purpose node manipulation functions:
copyfuncs.c - copy a node tree
equalfuncs.c - compare two node trees
outfuncs.c - convert a node tree to text representation
readfuncs.c - convert text representation back to a node tree
makefuncs.c - creator functions for some common node types
nodeFuncs.c - some other general-purpose manipulation functions
Specialized manipulation functions:
bitmapset.c - Bitmapset support
list.c - generic list support
params.c - Param support
tidbitmap.c - TIDBitmap support
value.c - support for Value nodes
FILES IN src/include/nodes/
Node definitions:
nodes.h - define node tags (NodeTag)
primnodes.h - primitive nodes
parsenodes.h - parse tree nodes
pathnodes.h - path tree nodes and planner internal structures
plannodes.h - plan tree nodes
execnodes.h - executor nodes
memnodes.h - memory nodes
pg_list.h - generic list
Steps to Add a Node
-------------------
Suppose you want to define a node Foo:
1. Add a tag (T_Foo) to the enum NodeTag in nodes.h. (If you insert the
tag in a way that moves the numbers associated with existing tags,
you'll need to recompile the whole tree after doing this. It doesn't
force initdb though, because the numbers never go to disk.)
2. Add the structure definition to the appropriate include/nodes/???.h file.
If you intend to inherit from, say a Plan node, put Plan as the first field
of your struct definition.
3. If you intend to use copyObject, equal, nodeToString or stringToNode,
add an appropriate function to copyfuncs.c, equalfuncs.c, outfuncs.c
and readfuncs.c accordingly. (Except for frequently used nodes, don't
bother writing a creator function in makefuncs.c) The header comments
in those files give general rules for whether you need to add support.
4. Add cases to the functions in nodeFuncs.c as needed. There are many
other places you'll probably also need to teach about your new node
type. Best bet is to grep for references to one or two similar existing
node types to find all the places to touch.
Historical Note
---------------
Prior to the current simple C structure definitions, the Node structures
used a pseudo-inheritance system which automatically generated creator and
accessor functions. Since every node inherited from LispValue, the whole thing
was a mess. Here's a little anecdote:
LispValue definition -- class used to support lisp structures
in C. This is here because we did not want to totally rewrite
planner and executor code which depended on lisp structures when
we ported postgres V1 from lisp to C. -cim 4/23/90