On 08/31/2015 10:07 AM, Kevin Grittner wrote:
Kevin,I've started to do a review on this patch but I am a bit confused with some of what I am seeing.
The attached testcase fails I replace the cursor in your test case with direct selects from the table. I would have expected this to generate the snapshot too old error as well but it doesn't.
# Failed test 'expect "snapshot too old" error' # at t/002_snapshot_too_old_select.pl line 64. # got: '' # expected: '72000' # Looks like you failed 1 test of 9. Dubious, test returned 1 (wstat 256, 0x100) Failed 1/9 subtests Am I misunderstanding something or is the patch not working as expected?
As discussed when the "proof of concept" patch was submitted during 9.5 development, here is a version intended to be considered for commit to 9.6, with the following changes: 1. It is configured using time rather than number of transactions. Not only was there unanimous agreement here that this was better, but the EDB customer who had requested this feature and who had been testing it independently made the same request. 2. The "proof of concept" patch only supported heap and btree checking; this supports all index types. 3. Documentation has been added. 4. Tests have been added. They are currently somewhat minimal, since this is using a whole new technique for testing from any existing committed tests -- I wanted to make sure that this approach to testing was OK with everyone before expanding it. If it is, I assume we will want to move some of the more generic portions to a .pm file to make it available for other tests. Basically, this patch aims to limit bloat when there are snapshots that are kept registered for prolonged periods. The immediate reason for this is a customer application that keeps read-only cursors against fairly static data open for prolonged periods, and automatically fields SQLSTATE 72000 to re-open them if necessary. When used, it should also provide some protections against extreme bloat from forgotten "idle in transaction" connections which are left holding a snapshot. Once a snapshot reaches the age threshold, it can be terminated if reads data modified after the snapshot was built. It is expected that useful ranges will normally be somewhere from a few hours to a few days. By default old_snapshot_threshold is set to -1, which disables the new behavior. The customer has been testing a preliminary version of this time-based patch for several weeks, and is happy with the results -- it is preventing bloat for them and not generating "snapshot too old" errors at unexpected times. -- Kevin Grittner EDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
002_snapshot_too_old_select.pl
Description: Perl program
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