Re: Reg: Help to understand the source code

On 4/23/20 8:44 AM, Preethi S wrote:
> I am fairly new to postgres and I am trying to understand how the data
> is processed during the insert from buffer to the disk. Can someone help
> me with that? Also, I would like to see source code workflow. Can
> someone help me with finding the source code for the data
> insertion/modification workflow.

I'm also a Postgres hacker newbie, but I've spent some time adding
SQL:2011 FOR PORTION OF support to UPDATE/DELETE, so I've gone through
that learning process. (I should say "going through". :-)

I'd say be prepared to spend a *lot* of time reading the code.
Personally I use `grep -r` a lot and just read and read. For specifics
you can use a debugger or insert `ereport(NOTICE, (errmsg("something
%s", foo)))` and run queries (or the test suite). Also many subfolders
have an extensive README that will guide you. Some of the READMEs may
take an hour or more to get through and understand, but reading them is
worth it.

It helped me a lot to spend several years writing occasional Postgres C
extensions before really doing anything in the core codebase. There are
lots of basics you learn that way. There are a bunch of articles and
presentations out there about that you might find helpful.

Postgres processes queries in several steps:

- parse
- analyze
- rewrite
- plan
- optimize
- execute

The parse step is a bison grammar (look for gram.y). Basically it fills
in structs cutting up what the user typed.

The analyze step starts to make sense of the parse results. Look at
parser/analyze.c. It maps input strings to database objects---for
example looking up table/column names (and making sure they really
exist). Here you're sort of just copying things from the parse structs
to different structs. You're building up Node trees that later steps can
use. I think the analyze step is often considered to be still part of
the parse phase.

It seems like each SQL "clause" has its own transformFoo function, so
probably you'll want to add your own (transformMyAwesomeFeatureClause)
and then call it from its "parent" (e.g. transformUpdateStmt).

If you add new Node types you'll need to edit nodes/*funcs.c and also
probably teach some switch statements how to handle them. If you are
filling in a struct but then later in the pipeline find that what you
wrote isn't there anymore, you probably forgot to implement a copy function.

The rewrite/plan/optimize steps aren't things you need to worry about
too much if you're interested in DML, but you can read more about them
in the source code. Especially rewrite is pretty niche (views and RULEs).

The execute step is the most challenging I think. It has its own Node
trees and also keeps an execution state. Probably you'll need to look at
src/backend/executor/nodeModifyTable.c among others. You'll also need to
learn about TupleTableSlots. (If anyone here has a good learning
resource for TTS I would also be glad to read it.)

I'm afraid this description is comically dumbed down, but hopefully it
can be something like a map. I'd probably just take an UPDATE statement
and try to trace it through the pipeline, and maybe experiment with
small changes along the way. You can add things to src/test/regress as
you go.

And the mailing list is a very friendly place to ask questions.

Yours,

--
Paul ~{:-)
pj(at)illuminatedcomputing(dot)com