On 12/9/19 7:35 AM, Julien Delplanque wrote:
Hello PostgreSQL hackers,
I hope I am posting on the right mailing-list.
I am actually doing a PhD related to relational databases and software
engineering.
I use PostgreSQL for my research.
I have a few questions about the internals of PostgreSQL and I think
they require experts knowledge.
I could not find documentation about that in the nice PostgreSQL
documentation but maybe I missed something? Tell me if it is the case.
My Questions:
Q1. Are PostgreSQL's meta-description tables (such as pg_class) the
"reality" concerning the state of the DB or are they just a virtual
representation ?
Not all of them are real tables; some of the pg_catalog relations are
views over others of them. But many of them are real tables with C
structs that back them. Take a look in src/include/catalog/pg_class.h
and you'll see the C struct definition, somewhat obscured by some
macros that make it less obvious to people not familiar with the
postgresql sources.
On line 29:
CATALOG(pg_class,1259,RelationRelationId) BKI_BOOTSTRAP
BKI_ROWTYPE_OID(83,RelationRelation_Rowtype_Id) BKI_SCHEMA_MACRO
{
...
}
That's a typedef. See genbki.h where it defines the macro:
#define CATALOG(name,oid,oidmacro) typedef struct CppConcat(FormData_,name)
What I would like to know with this question is: would it be possible to
implement DDL queries (e.g. CREATE TABLE, DROP TABLE, CREATE VIEW, ALTER
TABLE, etc.) as DML queries that modify the meta-data stored in
meta-description tables?
For example, something like:
INSERT INTO pg_class [...];
To create a new table (instead of the CREATE TABLE DDL query).
You are not allowed to insert into the pg_class table directly. There
are good reasons for that. Simply inserting a row into this table would
not cause all the infrastructure that backs a table to pop into
existence. So you have to use the DDL commands.
Q1.1 If it is possible, is what is done in reality? I have the feeling
that it is not the case and that DDL queries are implemented in C directly.
See src/backend/commands/tablecmds.c, function DefineRelation.
--
Mark Dilger
