Re: SKIP LOCKED DATA (work in progress)

Hello

As a simple example for people wondering what the point of this
feature is, I created a table work (id, data, status)
and then create 10,000 items with status 'NEW' and then started
a number of worker threads that did the following pair of
transactions, with and without SKIP LOCKED DATA on the end of the
SELECT statement, until all rows were deleted:

BEGIN
SELECT id, data FROM work WHERE status = 'NEW' LIMIT 1 FOR UPDATE
-- if no rows returned, then finish
UPDATE work SET status = 'WORK' WHERE id = $id
COMMIT

BEGIN
DELETE FROM work WHERE id = $id
COMMIT

Here are the times taken to process all items, in elapsed seconds, on
a slow laptop (i5 4 core with an SSD, with default postgresql.conf
except for checkpoint_segments set to 300):

| Threads | default | SKIP |
| 1 | 26.78 | 27.02 |
| 2 | 23.46 | 22.00 |
| 3 | 22.02 | 14.83 |
| 4 | 22.59 | 11.16 |
| 5 | 22.37 | 9.05 |
| 6 | 22.55 | 7.66 |
| 7 | 22.46 | 6.69 |
| 8 | 22.57 | 8.39 |
| 9 | 22.40 | 8.38 |
| 10 | 22.38 | 7.93 |
| 11 | 22.43 | 6.86 |
| 12 | 22.34 | 6.77 |

I am not experienced at benchmarking and I don't claim that this
particular workload or configuration is particularly sensible or
representative of anything but it might give some idea of the
motivation.

Best regards,
Thomas Munro