Re: Questions about PostgreSQL implementation details

Thanks for your answer, I will dive into the C code then.

Le 9/12/19 à 16:52, Mark Dilger a écrit :
>
> 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.

Indeed, I should have say it in my email, I saw in the documentation
that some of them are views. :-)

Cheers,

Julien

Le 9/12/19 à 16:52, Mark Dilger a écrit :
>
>
> 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.
>
>
>