Why avoid long transactions?

The majority of PostgreSQL community clearly understands why long and idle transactions are “bad”. But when you talk about it to the newcomers it’s always a good idea to backup your explanation with some real tests.

While preparing slides for my presentation about vacuum I have made a simple test case with long transaction using pgbench. Here are the results.

pgbench -c8 -P 60 -T 3600 -U postgres pgbench
starting vacuum...end.
progress: 60.0 s, 9506.3 tps, lat 0.841 ms stddev 0.390
progress: 120.0 s, 5262.1 tps, lat 1.520 ms stddev 0.517
progress: 180.0 s, 3801.8 tps, lat 2.104 ms stddev 0.757
progress: 240.0 s, 2960.0 tps, lat 2.703 ms stddev 0.830
progress: 300.0 s, 2575.8 tps, lat 3.106 ms stddev 0.891


in the end

progress: 3300.0 s, 759.5 tps, lat 10.533 ms stddev 2.554
progress: 3360.0 s, 751.8 tps, lat 10.642 ms stddev 2.604
progress: 3420.0 s, 743.6 tps, lat 10.759 ms stddev 2.655
progress: 3480.0 s, 739.1 tps, lat 10.824 ms stddev 2.662
progress: 3540.0 s, 742.5 tps, lat 10.774 ms stddev 2.579
progress: 3600.0 s, 868.2 tps, lat 9.215 ms stddev 2.569


This is a standard TPC-B pgbench test, running on a small database which completely resides in shared buffers (it removes disk IO influences).

As you can see, the performance measured in transaction per second initially dropped during the first few minutes of the test and continues to reduce further.

Look at the statistics from the vacuum logs:

tuples: 0 removed, 692428 remain, 691693 are dead but not yet removable, oldest xmin: 62109160
tuples: 0 removed, 984009 remain, 983855 are dead but not yet removable, oldest xmin: 62109160
tuples: 0 removed, 1176821 remain, 1176821 are dead but not yet removable, oldest xmin: 62109160
tuples: 0 removed, 1494122 remain, 1494122 are dead but not yet removable, oldest xmin: 62109160
tuples: 0 removed, 2022284 remain, 2022284 are dead but not yet removable, oldest xmin: 62109160
tuples: 0 removed, 2756298 remain, 2756153 are dead but not yet removable, oldest xmin: 62109160
tuples: 0 removed, 3500913 remain, 3500693 are dead but not yet removable, oldest xmin: 62109160
tuples: 0 removed, 4631448 remain, 4631354 are dead but not yet removable, oldest xmin: 62109160
tuples: 0 removed, 5377941 remain, 5374941 are dead but not yet removable, oldest xmin: 62109160


Number of dead rows that aren't cleaned up has increased over time, which means that vacuum is not working properly.

So, if you ever need proof for why long transactions are bad for your database, here you have it!

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