SELECT FOR UPDATE SKIP LOCKED in Postgresql is not skipping a record that should be skipped

kmitov
October 4, 2024
93 views
1 vote
2 Answers

I would like to build a queue of Articles where each article has a last_seen_at.
User arrives on web page, server ORDERs Articles by last_seen_at ASC, selects the first and returns to the browser. The server updates last_seen_at to be NOW() before returning to the browser.

In this way an Article is moved as last in the queue if it was just delivered.

For this I use FOR UPDATE SKIP LOCKED.

The problem is that under heavy load and concurrent request in a few milliseconds we return the same article to two different users.

The query is

SELECT * INTO rec
                FROM articles
                ORDER BY last_seen_at ASC NULLS FIRST
                LIMIT 1
                FOR UPDATE SKIP LOCKED;

                UPDATE articles SET last_seen_at = NOW() WHERE id = rec.id;

and the way to reproduce it is by running this script. It simulates 10 different clients that are executing requests with psql. Actual servers have more, so I tried to bring the script to bash, psql and SQL.

The idea is that 10 clients will request from the table of 20 records at almost the same time and each client should receive a unique article to read. There should be no duplicates.

#!/bin/bash

# Define database variables
DB_NAME="article_db"
DB_USER="...fill here..."
DB_HOST="localhost"    # Adjust as needed
DB_PORT="5432"         # Adjust as needed

# Drop the database
echo "Dropping database: $DB_NAME"
psql -U $DB_USER -c "DROP DATABASE IF EXISTS $DB_NAME;" postgres

echo "Database $DB_NAME has been removed."

# Create the database
echo "Creating database: $DB_NAME"
psql -U $DB_USER -c "CREATE DATABASE $DB_NAME;" postgres


psql -U $DB_USER -c "

    CREATE TABLE articles (
        id SERIAL PRIMARY KEY,
        title VARCHAR(255),
        content TEXT,
        last_seen_at TIMESTAMPTZ,
        created_at TIMESTAMPTZ DEFAULT NOW(),
        updated_at TIMESTAMPTZ DEFAULT NOW()
    );

    INSERT INTO articles (title, content, last_seen_at)
        VALUES
          ('Article 1', 'This is the content of article 1.', NOW()),
          ('Article 2', 'This is the content of article 2.', NOW()),
          ('Article 3', 'This is the content of article 3.', NOW()),
          ('Article 4', 'This is the content of article 4.', NOW()),
          ('Article 5', 'This is the content of article 5.', NOW()),
          ('Article 6', 'This is the content of article 6.', NOW()),
          ('Article 7', 'This is the content of article 7.', NOW()),
          ('Article 8', 'This is the content of article 8.', NOW()),
          ('Article 9', 'This is the content of article 9.', NOW()),
          ('Article 10', 'This is the content of article 10.', NOW()),
          ('Article 11', 'This is the content of article 11.', NOW()),
          ('Article 12', 'This is the content of article 12.', NOW()),
          ('Article 13', 'This is the content of article 13.', NOW()),
          ('Article 14', 'This is the content of article 14.', NOW()),
          ('Article 15', 'This is the content of article 15.', NOW()),
          ('Article 16', 'This is the content of article 16.', NOW()),
          ('Article 17', 'This is the content of article 17.', NOW()),
          ('Article 18', 'This is the content of article 18.', NOW()),
          ('Article 19', 'This is the content of article 19.', NOW()),
          ('Article 20', 'This is the content of article 20.', NOW());

" $DB_NAME

echo "" > output.txt
echo $(psql --version) >> output.txt
for i in {0..15}; do
    echo "Run $i" >> output.txt
    x=0.01
    y=0.2
    sleep_time=$(awk -v min=$x -v max=$y 'BEGIN{srand(); print min+rand()*(max-min)}')
    sleep $sleep_time # just so that NOW() returns different values and not close to each other

    for n in {0..9}; do
        psql -U $DB_USER -c "
            DO $$
            DECLARE
                rec RECORD;
            BEGIN
                -- Select and lock the row
                SELECT * INTO rec
                FROM articles
                ORDER BY last_seen_at ASC NULLS FIRST
                LIMIT 1
                FOR UPDATE SKIP LOCKED;

                UPDATE articles SET last_seen_at = NOW() WHERE id = rec.id;

                -- Return the result after commit
                RAISE NOTICE 'ID: %, Title: %, Last: %', rec.id, rec.title, rec.last_seen_at;
            END $$;
        " $DB_NAME 2>> output.txt &
    done
    wait
    echo "Run $i done"
done

When you run the script what happens is


psql (PostgreSQL) 12.12
Run 0
NOTICE:  ID: 1, Title: Article 1, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 2, Title: Article 2, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 3, Title: Article 3, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 4, Title: Article 4, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 5, Title: Article 5, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 6, Title: Article 6, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 7, Title: Article 7, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 8, Title: Article 8, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 9, Title: Article 9, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 10, Title: Article 10, Last: 2024-10-03 23:07:55.191153+03
Run 1
NOTICE:  ID: 11, Title: Article 11, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 12, Title: Article 12, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 13, Title: Article 13, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 14, Title: Article 14, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 15, Title: Article 15, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 16, Title: Article 16, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 18, Title: Article 18, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 19, Title: Article 19, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 20, Title: Article 20, Last: 2024-10-03 23:07:55.191153+03
NOTICE:  ID: 17, Title: Article 17, Last: 2024-10-03 23:07:55.191153+03
Run 2
NOTICE:  ID: 4, Title: Article 4, Last: 2024-10-03 23:07:55.357+03
NOTICE:  ID: 1, Title: Article 1, Last: 2024-10-03 23:07:55.357631+03
NOTICE:  ID: 2, Title: Article 2, Last: 2024-10-03 23:07:55.360134+03
NOTICE:  ID: 3, Title: Article 3, Last: 2024-10-03 23:07:55.366221+03
NOTICE:  ID: 5, Title: Article 5, Last: 2024-10-03 23:07:55.369527+03
NOTICE:  ID: 6, Title: Article 6, Last: 2024-10-03 23:07:55.370643+03
NOTICE:  ID: 7, Title: Article 7, Last: 2024-10-03 23:07:55.379598+03
NOTICE:  ID: 8, Title: Article 8, Last: 2024-10-03 23:07:55.38012+03
NOTICE:  ID: 9, Title: Article 9, Last: 2024-10-03 23:07:55.380929+03
NOTICE:  ID: 10, Title: Article 10, Last: 2024-10-03 23:07:55.381026+03
Run 3
NOTICE:  ID: 11, Title: Article 11, Last: 2024-10-03 23:07:55.529283+03
NOTICE:  ID: 12, Title: Article 12, Last: 2024-10-03 23:07:55.531521+03
NOTICE:  ID: 13, Title: Article 13, Last: 2024-10-03 23:07:55.53443+03
NOTICE:  ID: 14, Title: Article 14, Last: 2024-10-03 23:07:55.539379+03
NOTICE:  ID: 15, Title: Article 15, Last: 2024-10-03 23:07:55.544098+03
NOTICE:  ID: 16, Title: Article 16, Last: 2024-10-03 23:07:55.545991+03
NOTICE:  ID: 16, Title: Article 16, Last: 2024-10-03 23:07:55.889783+03
NOTICE:  ID: 18, Title: Article 18, Last: 2024-10-03 23:07:55.552443+03
NOTICE:  ID: 19, Title: Article 19, Last: 2024-10-03 23:07:55.55334+03
NOTICE:  ID: 20, Title: Article 20, Last: 2024-10-03 23:07:55.553517+03
Run 4
NOTICE:  ID: 17, Title: Article 17, Last: 2024-10-03 23:07:55.553575+03
NOTICE:  ID: 4, Title: Article 4, Last: 2024-10-03 23:07:55.704585+03
NOTICE:  ID: 1, Title: Article 1, Last: 2024-10-03 23:07:55.706418+03
NOTICE:  ID: 2, Title: Article 2, Last: 2024-10-03 23:07:55.709828+03
NOTICE:  ID: 3, Title: Article 3, Last: 2024-10-03 23:07:55.711804+03
NOTICE:  ID: 5, Title: Article 5, Last: 2024-10-03 23:07:55.714998+03
NOTICE:  ID: 6, Title: Article 6, Last: 2024-10-03 23:07:55.720595+03
NOTICE:  ID: 7, Title: Article 7, Last: 2024-10-03 23:07:55.72123+03
NOTICE:  ID: 8, Title: Article 8, Last: 2024-10-03 23:07:55.721629+03
NOTICE:  ID: 10, Title: Article 10, Last: 2024-10-03 23:07:55.721857+03
Run 5
NOTICE:  ID: 9, Title: Article 9, Last: 2024-10-03 23:07:55.721988+03
NOTICE:  ID: 12, Title: Article 12, Last: 2024-10-03 23:07:55.871198+03
NOTICE:  ID: 11, Title: Article 11, Last: 2024-10-03 23:07:55.871329+03
NOTICE:  ID: 14, Title: Article 14, Last: 2024-10-03 23:07:55.878223+03
NOTICE:  ID: 13, Title: Article 13, Last: 2024-10-03 23:07:55.873865+03
NOTICE:  ID: 15, Title: Article 15, Last: 2024-10-03 23:07:55.881956+03
NOTICE:  ID: 16, Title: Article 16, Last: 2024-10-03 23:07:55.890476+03
NOTICE:  ID: 18, Title: Article 18, Last: 2024-10-03 23:07:55.890876+03
NOTICE:  ID: 19, Title: Article 19, Last: 2024-10-03 23:07:55.892448+03
NOTICE:  ID: 20, Title: Article 20, Last: 2024-10-03 23:07:55.892666+03
Run 6
NOTICE:  ID: 17, Title: Article 17, Last: 2024-10-03 23:07:56.045753+03
NOTICE:  ID: 4, Title: Article 4, Last: 2024-10-03 23:07:56.046676+03
NOTICE:  ID: 2, Title: Article 2, Last: 2024-10-03 23:07:56.053627+03
NOTICE:  ID: 1, Title: Article 1, Last: 2024-10-03 23:07:56.049792+03
NOTICE:  ID: 3, Title: Article 3, Last: 2024-10-03 23:07:56.059395+03
NOTICE:  ID: 5, Title: Article 5, Last: 2024-10-03 23:07:56.062333+03
NOTICE:  ID: 7, Title: Article 7, Last: 2024-10-03 23:07:56.064246+03
NOTICE:  ID: 6, Title: Article 6, Last: 2024-10-03 23:07:56.064377+03
NOTICE:  ID: 8, Title: Article 8, Last: 2024-10-03 23:07:56.064599+03
NOTICE:  ID: 10, Title: Article 10, Last: 2024-10-03 23:07:56.06874+03
Run 7
NOTICE:  ID: 12, Title: Article 12, Last: 2024-10-03 23:07:56.303647+03
NOTICE:  ID: 9, Title: Article 9, Last: 2024-10-03 23:07:56.304069+03
NOTICE:  ID: 11, Title: Article 11, Last: 2024-10-03 23:07:56.310685+03
NOTICE:  ID: 14, Title: Article 14, Last: 2024-10-03 23:07:56.314391+03
NOTICE:  ID: 13, Title: Article 13, Last: 2024-10-03 23:07:56.314454+03
NOTICE:  ID: 15, Title: Article 15, Last: 2024-10-03 23:07:56.315747+03
NOTICE:  ID: 15, Title: Article 15, Last: 2024-10-03 23:07:56.8011+03
NOTICE:  ID: 18, Title: Article 18, Last: 2024-10-03 23:07:56.317306+03
NOTICE:  ID: 16, Title: Article 16, Last: 2024-10-03 23:07:56.317362+03
NOTICE:  ID: 19, Title: Article 19, Last: 2024-10-03 23:07:56.317796+03
Run 8
NOTICE:  ID: 20, Title: Article 20, Last: 2024-10-03 23:07:56.317888+03
NOTICE:  ID: 17, Title: Article 17, Last: 2024-10-03 23:07:56.546232+03
NOTICE:  ID: 1, Title: Article 1, Last: 2024-10-03 23:07:56.556373+03
NOTICE:  ID: 4, Title: Article 4, Last: 2024-10-03 23:07:56.549424+03
NOTICE:  ID: 2, Title: Article 2, Last: 2024-10-03 23:07:56.559143+03
NOTICE:  ID: 4, Title: Article 4, Last: 2024-10-03 23:07:57.028894+03
NOTICE:  ID: 3, Title: Article 3, Last: 2024-10-03 23:07:56.560479+03
NOTICE:  ID: 5, Title: Article 5, Last: 2024-10-03 23:07:56.561629+03
NOTICE:  ID: 7, Title: Article 7, Last: 2024-10-03 23:07:56.563405+03
NOTICE:  ID: 6, Title: Article 6, Last: 2024-10-03 23:07:56.564448+03
Run 9
NOTICE:  ID: 8, Title: Article 8, Last: 2024-10-03 23:07:56.565825+03
NOTICE:  ID: 10, Title: Article 10, Last: 2024-10-03 23:07:56.566152+03
NOTICE:  ID: 12, Title: Article 12, Last: 2024-10-03 23:07:56.783754+03
NOTICE:  ID: 9, Title: Article 9, Last: 2024-10-03 23:07:56.783821+03
NOTICE:  ID: 11, Title: Article 11, Last: 2024-10-03 23:07:56.791792+03
NOTICE:  ID: 13, Title: Article 13, Last: 2024-10-03 23:07:56.792547+03
NOTICE:  ID: 14, Title: Article 14, Last: 2024-10-03 23:07:56.79418+03
NOTICE:  ID: 14, Title: Article 14, Last: 2024-10-03 23:07:57.169286+03
NOTICE:  ID: 15, Title: Article 15, Last: 2024-10-03 23:07:56.801556+03
NOTICE:  ID: 18, Title: Article 18, Last: 2024-10-03 23:07:56.801921+03
Run 10
NOTICE:  ID: 16, Title: Article 16, Last: 2024-10-03 23:07:56.80328+03
NOTICE:  ID: 19, Title: Article 19, Last: 2024-10-03 23:07:56.803508+03
NOTICE:  ID: 20, Title: Article 20, Last: 2024-10-03 23:07:57.021593+03
NOTICE:  ID: 17, Title: Article 17, Last: 2024-10-03 23:07:57.02499+03
NOTICE:  ID: 4, Title: Article 4, Last: 2024-10-03 23:07:57.033416+03
NOTICE:  ID: 1, Title: Article 1, Last: 2024-10-03 23:07:57.036014+03
NOTICE:  ID: 2, Title: Article 2, Last: 2024-10-03 23:07:57.039942+03
NOTICE:  ID: 5, Title: Article 5, Last: 2024-10-03 23:07:57.043892+03
NOTICE:  ID: 3, Title: Article 3, Last: 2024-10-03 23:07:57.044432+03
NOTICE:  ID: 5, Title: Article 5, Last: 2024-10-03 23:07:57.295768+03
Run 11
NOTICE:  ID: 6, Title: Article 6, Last: 2024-10-03 23:07:57.046077+03
NOTICE:  ID: 10, Title: Article 10, Last: 2024-10-03 23:07:57.144911+03
NOTICE:  ID: 7, Title: Article 7, Last: 2024-10-03 23:07:57.045291+03
NOTICE:  ID: 8, Title: Article 8, Last: 2024-10-03 23:07:57.146934+03
NOTICE:  ID: 12, Title: Article 12, Last: 2024-10-03 23:07:57.151276+03
NOTICE:  ID: 9, Title: Article 9, Last: 2024-10-03 23:07:57.157833+03
NOTICE:  ID: 11, Title: Article 11, Last: 2024-10-03 23:07:57.163592+03
NOTICE:  ID: 13, Title: Article 13, Last: 2024-10-03 23:07:57.167025+03
NOTICE:  ID: 14, Title: Article 14, Last: 2024-10-03 23:07:57.169716+03
NOTICE:  ID: 15, Title: Article 15, Last: 2024-10-03 23:07:57.170998+03
Run 12
NOTICE:  ID: 18, Title: Article 18, Last: 2024-10-03 23:07:57.171116+03
NOTICE:  ID: 16, Title: Article 16, Last: 2024-10-03 23:07:57.275763+03
NOTICE:  ID: 19, Title: Article 19, Last: 2024-10-03 23:07:57.282054+03
NOTICE:  ID: 20, Title: Article 20, Last: 2024-10-03 23:07:57.289475+03
NOTICE:  ID: 17, Title: Article 17, Last: 2024-10-03 23:07:57.292438+03
NOTICE:  ID: 4, Title: Article 4, Last: 2024-10-03 23:07:57.294022+03
NOTICE:  ID: 1, Title: Article 1, Last: 2024-10-03 23:07:57.2951+03
NOTICE:  ID: 2, Title: Article 2, Last: 2024-10-03 23:07:57.295277+03
NOTICE:  ID: 3, Title: Article 3, Last: 2024-10-03 23:07:57.295999+03
NOTICE:  ID: 5, Title: Article 5, Last: 2024-10-03 23:07:57.29623+03
Run 13
NOTICE:  ID: 8, Title: Article 8, Last: 2024-10-03 23:07:57.393965+03
NOTICE:  ID: 7, Title: Article 7, Last: 2024-10-03 23:07:57.395886+03
NOTICE:  ID: 6, Title: Article 6, Last: 2024-10-03 23:07:57.398183+03
NOTICE:  ID: 10, Title: Article 10, Last: 2024-10-03 23:07:57.404+03
NOTICE:  ID: 12, Title: Article 12, Last: 2024-10-03 23:07:57.410518+03
NOTICE:  ID: 13, Title: Article 13, Last: 2024-10-03 23:07:57.419476+03
NOTICE:  ID: 9, Title: Article 9, Last: 2024-10-03 23:07:57.419919+03
NOTICE:  ID: 11, Title: Article 11, Last: 2024-10-03 23:07:57.420215+03
NOTICE:  ID: 14, Title: Article 14, Last: 2024-10-03 23:07:57.421694+03
NOTICE:  ID: 15, Title: Article 15, Last: 2024-10-03 23:07:57.421805+03
Run 14
NOTICE:  ID: 18, Title: Article 18, Last: 2024-10-03 23:07:57.522213+03
NOTICE:  ID: 16, Title: Article 16, Last: 2024-10-03 23:07:57.524602+03
NOTICE:  ID: 19, Title: Article 19, Last: 2024-10-03 23:07:57.531331+03
NOTICE:  ID: 20, Title: Article 20, Last: 2024-10-03 23:07:57.533905+03
NOTICE:  ID: 17, Title: Article 17, Last: 2024-10-03 23:07:57.538787+03
NOTICE:  ID: 4, Title: Article 4, Last: 2024-10-03 23:07:57.53948+03
NOTICE:  ID: 1, Title: Article 1, Last: 2024-10-03 23:07:57.540906+03
NOTICE:  ID: 3, Title: Article 3, Last: 2024-10-03 23:07:57.540984+03
NOTICE:  ID: 2, Title: Article 2, Last: 2024-10-03 23:07:57.541091+03
NOTICE:  ID: 5, Title: Article 5, Last: 2024-10-03 23:07:57.541513+03
Run 15
NOTICE:  ID: 8, Title: Article 8, Last: 2024-10-03 23:07:57.642186+03
NOTICE:  ID: 6, Title: Article 6, Last: 2024-10-03 23:07:57.645733+03
NOTICE:  ID: 7, Title: Article 7, Last: 2024-10-03 23:07:57.6465+03
NOTICE:  ID: 10, Title: Article 10, Last: 2024-10-03 23:07:57.654263+03
NOTICE:  ID: 12, Title: Article 12, Last: 2024-10-03 23:07:57.659976+03
NOTICE:  ID: 13, Title: Article 13, Last: 2024-10-03 23:07:57.661056+03
NOTICE:  ID: 9, Title: Article 9, Last: 2024-10-03 23:07:57.661537+03
NOTICE:  ID: 11, Title: Article 11, Last: 2024-10-03 23:07:57.662008+03
NOTICE:  ID: 14, Title: Article 14, Last: 2024-10-03 23:07:57.663009+03
NOTICE:  ID: 15, Title: Article 15, Last: 2024-10-03 23:07:57.663025+03

We do 15 runs and in this 15 runs of requesting 10 articles in parallel, we get:
In run 3, article 16 was returned two times

NOTICE:  ID: 16, Title: Article 16, Last: 2024-10-03 23:07:55.545991+03
NOTICE:  ID: 16, Title: Article 16, Last: 2024-10-03 23:07:55.889783+03

In run 7 article 15 was returned two times

NOTICE:  ID: 15, Title: Article 15, Last: 2024-10-03 23:07:56.315747+03
NOTICE:  ID: 15, Title: Article 15, Last: 2024-10-03 23:07:56.8011+03

In run 8 article 4 was returned two times

NOTICE:  ID: 4, Title: Article 4, Last: 2024-10-03 23:07:56.549424+03
NOTICE:  ID: 2, Title: Article 2, Last: 2024-10-03 23:07:56.559143+03
NOTICE:  ID: 4, Title: Article 4, Last: 2024-10-03 23:07:57.028894+03

and so on.

It happens about 5-10 percent of the cases.

What I fail to understand is why FOR UPDATE SKIP LOCKED is not working the way I would expect it to work – lock a row and if others try to lock it they should move to a different row.

Thanks

Tags: postgresql sql

Answers

- Zegarek
- October 3, 2024 at 11:39 pm
- 0 votes
0
In each run you’re deploying the 10 psql instances and pushing them to background, in sequence:
```
for n in {0..9}; do
        psql -U $DB_USER -c "/*Script*/" $DB_NAME 2>> output.txt &
done
```
They do not start all at once so there’s a chance any two following workers catch the same row, one worker after another, just because the earlier one commits before the later one even runs the select. The select..for update gets you a transaction-level lock:

Row-level locks are released at transaction end or during savepoint rollback, just like table-level locks.

That lock might be already lifted before the later worker arrives.

If at that point the row also just happens to still hold the earliest last_seen_at, even after bumping up its timestamp, the worker is forced to pick it: order by demands it, and it’s no longer locked for for update skip locked to skip it.

As a result, two workers can get the same row in the same run, but that does not mean they’re processing it at the same time, or that locks are somehow disregarded.

Note how the second row 16 has a mich higher timestamp than all others in its run:

id last_seen_at

16 2024-10-03 23:07:55.545991+03

16 2024-10-03 23:07:55.889783+03

18 2024-10-03 23:07:55.552443+03

19 2024-10-03 23:07:55.55334+03

20 2024-10-03 23:07:55.553517+03

Same story with the other two "violators" – they arrived late enough to catch a row just released by an earlier, quicker commiter from that same run.

Tracking all this just based on the end result is tricky because:
1. You’re updating the timestamp to now() which is the current time as of the start of the transaction, and
  - not the time of the locking select
  - not the time of the subsequent update
  - not the time you RAISE NOTICE to write this in the log
  - not the commit at the end, when the lock’s lifted
  - not the script start or finish.
2. Even though all your workers perform the same task, it’s not guaranteed how much time it’ll take for any of them. The one that started first might finish last, or whenever.
3. Even though they are started in a specific order, they are not even guaranteed the 1st one manages to connect and cache a now() before the 10th.
4. Even if the 1st worker does manage to connect before the 10th worker, it might happen that the 10th connects later, gets a later now(), but beats the 1st one to running the select and locking a row.
5. The order in which they proceed to run their update may also be different.
6. The order in which they raise might be different.
7. The order in which the OS writes their NOTICE messages to the output.txt file might be different.
8. They commit and close the session at different times.
Watch it go back and forth in Run 6:

id last_seen_at

4 2024-10-03 23:07:56.046676+03

2 2024-10-03 23:07:56.053627+03

1 2024-10-03 23:07:56.049792+03

3 2024-10-03 23:07:56.059395+03

To make sure, you can take snapshots of your whole target table and pg_locks each worker sees and also collect somewhat more current clock_timestamp()s in addition to the transaction-starting now()s for more context:

clock_timestamp() returns the actual current time, and therefore its value changes even within a single SQL command. (…) now() is a traditional PostgreSQL equivalent to transaction_timestamp()

You can also add a pg_sleep() in there before taking the lock and another before completion to emulate something closer to true concurrency, so that workers are weighed down a bit, preventing earlier ones from finishing before the final ones even start.
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- jjanes
- October 4, 2024 at 6:24 pm
- 0 votes
0
I think you are seeing a result of this (from the docs):

It is possible for a SELECT command running at the READ
COMMITTED transaction isolation level and using ORDER BY and a locking
clause to return rows out of order. This is because ORDER BY is
applied first. The command sorts the result, but might then block
trying to obtain a lock on one or more of the rows. Once the SELECT
unblocks, some of the ordering column values might have been modified,
leading to those rows appearing to be out of order

So the ordering is applied with the old value of last_seen_at, but then when it goes to lock the first row, a new value has already been committed and it sees that post-commit value but doesn’t reapply the ORDER BY. (The doc description is in terms of blocking, which doesn’t literally apply here as it doesn’t actually block, instead finding the commit has already happened so no blocking was necessary)

There should be no duplicates.

But clearly there should be duplicates. You are applying 160 updates to only 20 rows, there has to be duplicates. The duplicates are not happening in the order you want them to, but that isn’t something you can control with the isolation level you are using. If you had actually prevented duplicates, like by setting a seen-already-flag in your UPDATE and including that flag test in your WHERE, then the mechanism would work to prevent duplicates.

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`id`	`last_seen_at`
16	2024-10-03 23:07:55.545991+03
16	2024-10-03 23:07:55.889783+03
18	2024-10-03 23:07:55.552443+03
19	2024-10-03 23:07:55.55334+03
20	2024-10-03 23:07:55.553517+03

`id`	`last_seen_at`
4	2024-10-03 23:07:56.046676+03
2	2024-10-03 23:07:56.053627+03
1	2024-10-03 23:07:56.049792+03
3	2024-10-03 23:07:56.059395+03

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