PostgreSQL – How to Determine Rows Modified by a Transaction Before It Ends

monitoringpostgresqlupdate

I have a ~320 million row table on which I run the following query:

UPDATE my_table SET state = TRIM(state)

Needless to say this has been running for 90+ hours (since thursday afternoon) and has not finished yet.

I'd like to know, more or less, how many rows it has modified until now.
Is there any way to obtain this information from the running transaction?

Trying to search I have found how to determine a rough estimate of how many new rows are being added to a table using the pgstattuple function:

select dead_tuple_count from pgstattuple('my_table');

But this does not seem to show any interesting information for how many rows have been modified.

Best Answer

There is at least one way to peek at the progress of an uncommitted UPDATE, though it's a bit heavy-handed.

Postgres handles transaction isolation through row versioning. Their implementation involves tagging every record version with the smallest and largest transaction IDs which are allowed to see it (xmin and xmax, respectively).

Under this scheme, an UPDATE works by setting the xmax of the target record to the current transaction ID (equivalent to a DELETE) and creating an updated copy with the transaction ID in xmin (equivalent to an INSERT).

These system columns can be queried, so given the transaction ID of the UPDATE (which you can get from pg_stat_activity.backend_xid), you can find out how many rows it's processed with e.g.:

SELECT COUNT(*)
FROM my_table
WHERE xmax = 2357

Things get a bit messier if the transaction has set any savepoints, in which case the xmax will be a subtransaction ID, which doesn't appear in pg_stat_activity (or anywhere else, as far as I'm aware). In that case, you can inspect all rows which have been marked for update/deletion, by either in-progress or rolled-back transactions, with:

SELECT xmax, COUNT(*)
FROM my_table
WHERE xmax <> 0
GROUP BY xmax

... and from there, it shouldn't be too hard to figure out which ID is the one you're interested in.

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