Horton

RE: row filtering for logical replication

Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>

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From: "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, Tomas Vondra <tomas.vondra@enterprisedb.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Greg Nancarrow <gregn4422@gmail.com>, Euler Taveira <euler@eulerto.com>, Peter Smith <smithpb2250@gmail.com>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, Önder Kalacı <onderkalaci@gmail.com>, japin <japinli@hotmail.com>, Michael Paquier <michael@paquier.xyz>, David Steele <david@pgmasters.net>, Craig Ringer <craig@2ndquadrant.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Amit Langote <amitlangote09@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Rahila Syed <rahilasyed90@gmail.com>
Date: 2021-07-27T00:51:41Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Release cache tuple when no longer needed

  2. Add some additional tests for row filters in logical replication.

  3. Fix one of the tests introduced in commit 52e4f0cd47.

  4. Allow specifying row filters for logical replication of tables.

  5. Move scanint8() to numutils.c

  6. Replace Test::More plans with done_testing

  7. Reduce relcache access in WAL sender streaming logical changes

  8. Small cleanups related to PUBLICATION framework code

  9. Add a view to show the stats of subscription workers.

  10. Allow publishing the tables of schema.

  11. Doc: improve documentation of CREATE/ALTER SUBSCRIPTION.

  12. Add PublicationTable and PublicationRelInfo structs

  13. Remove unused argument "txn" in maybe_send_schema().

  14. Add prepare API support for streaming transactions in logical replication.

  15. Unify PostgresNode's new() and get_new_node() methods

  16. Use l*_node() family of functions where appropriate

  17. Add support for prepared transactions to built-in logical replication.

  18. Restore the portal-level snapshot after procedure COMMIT/ROLLBACK.

  19. Rename a parse node to be more general

  20. Remove unused column atttypmod from initial tablesync query

  21. SEARCH and CYCLE clauses

On July 23, 2021 6:16 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> On Fri, Jul 23, 2021 at 2:27 PM Rahila Syed <rahilasyed90@gmail.com> wrote:
> >
> > The column comparison for row filtering happens before the unchanged
> > toast columns are filtered. Unchanged toast columns are filtered just
> > before writing the tuple to output stream.
> >
> 
> To perform filtering, you need to use the tuple from WAL and that tuple doesn't
> seem to have unchanged toast values, so how can we do filtering? I think it is a
> good idea to test this once.

I agreed.

Currently, both unchanged toasted key column and unchanged toasted non-key
column is not logged. So, we cannot get the toasted value directly for these
columns when doing row filtering.

I tested the current patch for toasted data and found a problem: In the current
patch, it will try to fetch the toast data from toast table when doing row
filtering[1]. But, it's unsafe to do that in walsender. We can see it use
HISTORIC snapshot in heap_fetch_toast_slice() and also the comments of
init_toast_snapshot() have said "Detoasting *must* happen in the same
transaction that originally fetched the toast pointer.". The toast data could
have been changed when doing row filtering. For exmaple, I tested the following
steps and get an error.

1) UPDATE a nonkey column in publisher.
2) Use debugger to block the walsender process in function
   pgoutput_row_filter_exec_expr().
3) Open another psql to connect the publisher, and drop the table which updated
   in 1).
4) Unblock the debugger in 2), and then I can see the following error:
---
ERROR:  could not read block 0 in file "base/13675/16391"
---

[1]
(1)------publisher------
CREATE TABLE toasted_key (
    id serial,
    toasted_key text PRIMARY KEY,
    toasted_col1 text,
    toasted_col2 text
);
select repeat('9999999999', 200) as tvalue \gset
CREATE PUBLICATION pub FOR TABLE toasted_key WHERE (toasted_col2 = :'tvalue');
ALTER TABLE toasted_key REPLICA IDENTITY USING INDEX toasted_key_pkey;
ALTER TABLE toasted_key ALTER COLUMN toasted_key SET STORAGE EXTERNAL;
ALTER TABLE toasted_key ALTER COLUMN toasted_col1 SET STORAGE EXTERNAL;
ALTER TABLE toasted_key ALTER COLUMN toasted_col2 SET STORAGE EXTERNAL;
INSERT INTO toasted_key(toasted_key, toasted_col1, toasted_col2) VALUES(repeat('1234567890', 200), repeat('9876543210', 200), repeat('9999999999', 200));

(2)------subscriber------
CREATE TABLE toasted_key (
    id serial,
    toasted_key text PRIMARY KEY,
    toasted_col1 text,
    toasted_col2 text
);

CREATE SUBSCRIPTION sub CONNECTION 'dbname=postgres port=10000' PUBLICATION pub;

(3)------publisher------
UPDATE toasted_key SET toasted_col1 = repeat('1111113113', 200);

Based on the above steps, the row filter will ge through the following path
and fetch toast data in walsender.
------
pgoutput_row_filter_exec_expr
    ...
    texteq
        ...
        text *targ1 = DatumGetTextPP(arg1);
            pg_detoast_datum_packed
                detoast_attr
------