On Thursday 28 October 2010 7:04:48 pm Karl Pickett wrote: > Hello Postgres Hackers, > > We have a simple 'event log' table that is insert only (by multiple > concurrent clients). It has an integer primary key. We want to do > incremental queries of this table every 5 minutes or so, i.e. "select > * from events where id > LAST_ID_I_GOT" to insert into a separate > reporting database. The problem is, this simple approach has a race > that will forever skip uncommitted events. I.e., if 5000 was > committed sooner than 4999, and we get 5000, we will never go back and > get 4999 when it finally commits. How can we solve this? Basically > it's a phantom row problem but it spans transactions. > > I looked at checking the internal 'xmin' column but the docs say that > is 32 bit, and something like 'txid_current_snapshot' returns a 64 bit > value. I don't get it. http://www.postgresql.org/docs/8.4/interactive/functions-info.html#FUNCTIONS-TXID-SNAPSHOT-PARTS "The internal transaction ID type (xid) is 32 bits wide and wraps around every 4 billion transactions. However, these functions export a 64-bit format that is extended with an "epoch" counter so it will not wrap around during the life of an installation. The data type used by these functions, txid_snapshot, stores information about transaction ID visibility at a particular moment in time. Its components are described in Table 9-53. " So: Current snapshot: test=> SELECT txid_current_snapshot(); txid_current_snapshot ----------------------- 5098:5098: xmin of snapshot: test=> SELECT txid_snapshot_xmin(txid_current_snapshot()); txid_snapshot_xmin -------------------- 5098 (1 row) > All I want to is make sure I skip over any > rows that are newer than the oldest currently running transaction. > Has nobody else run into this before? > > Thank you very much. > > -- > Karl Pickett -- Adrian Klaver adrian.klaver@xxxxxxxxx -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
