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* ALTER ... ADD COLUMN with defaults and NOT NULL constraints works per SQL spec. A default is implemented by rewriting the table with the new value stored in each row. * ALTER COLUMN TYPE. You can change a column's datatype to anything you want, so long as you can specify how to convert the old value. Rewrites the table. (Possible future improvement: optimize no-op conversions such as varchar(N) to varchar(N+1).) * Multiple ALTER actions in a single ALTER TABLE command. You can perform any number of column additions, type changes, and constraint additions with only one pass over the table contents. Basic documentation provided in ALTER TABLE ref page, but some more docs work is needed. Original patch from Rod Taylor, additional work from Tom Lane.
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* *
* EXPLANATION OF THE NODE STRUCTURES *
* - Andrew Yu (11/94) *
* *
* Copyright (c) 1994, Regents of the University of California *
* *
* $PostgreSQL: pgsql/src/backend/nodes/README,v 1.2 2003/11/29 22:39:45 pgsql Exp $
* *
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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
Node manipulation functions:
copyfuncs.c - copying a node
equalfuncs.c - comparing a node
outfuncs.c - convert a node to ascii representation
readfuncs.c - convert ascii representation back to a node
makefuncs.c - creator functions for primitive nodes
Node definitions:
nodes.h - define node tags (NodeTag)
pg_list.h - generic list
primnodes.h - primitive nodes
parsenodes.h - parse tree nodes
plannodes.h - plan tree nodes
relation.h - inner plan tree nodes
execnodes.h - executor nodes
memnodes.h - memory nodes
STEPS TO ADD A NODE
Suppose you wana define a node Foo:
1. add a tag (T_Foo) to the enum NodeTag in nodes.h (You may have to
recompile the whole tree after doing this.)
2. add the structure definition to the appropriate ???nodes.h file. If you
intend to inherit from, say a Plan node, put Plan as the first field of
you 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)
HISTORICAL NOTE
Prior to the current simple C structure definitions, the Node structures
uses a pseudo-inheritance system which automatically generates creator and
accessor functions. Since every node inherits from LispValue, the whole thing
is 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