On 10/04/15 18:03, Andres Freund wrote:
On 2015-04-07 17:08:16 +0200, Andres Freund wrote:
I'm starting benchmarks now.

What I'm benchmarking here is the WAL overhead, since that's what we're

The test setup I used was a pgbench scale 10 instance. I've run with
full_page_write=off to have more reproducible results. This of course
over-emphasizes the overhead a bit. But for a long checkpoint interval
and a memory resident working set it's not that unrealistic.

I ran 50k transactions in a signle b
- 20445024
- 20437128
- 20436864
- avg: 20439672
extern 2byte identifiers:
- 23318368
- 23148648
- 23128016
- avg: 23198344
- avg overhead: 13.5%
padding 2byte identifiers:
- 21160408
- 21319720
- 21164280
- avg: 21214802
- avg overhead: 3.8%
extern 4byte identifiers:
- 23514216
- 23540128
- 23523080
- avg: 23525808
- avg overhead: 15.1%

To me that shows pretty clearly that a) reusing the padding is
worthwhile b) even without that using 2byte instead of 4 byte
identifiers is beneficial.

My opinion is that 10% of WAL size difference is quite high price to pay so that we can keep the padding for some other, yet unknown feature that hasn't come up in several years, which would need those 2 bytes.

But if we are willing to pay it then we can really go all the way and just use Oids...

 Petr Jelinek                  http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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