I have two aws task where one could be writing to table another could run delete query at the same time both could be running parallel queries but playing with different rows. so the question is if I run delete from table where column_name=some condition ? then will postgres apply table level lock or row level lock. If table level lock gets applied then another task will not able to write into table.
PostgreSQL will only lock the rows you delete.
Concurrent INSERTs will not be affected.
There are many different lock modes. The table will be locked, but in a mode that still allows other INSERT, DELETE, and UPDATE operations to happen concurrently. The rows actually deleted will also be locked, in a more restrictive mode.
Related
I'm pretty new to PostgreSQL and I'm sure I'm missing something here.
The scenario is with version 11, executing a big drop table and insert transaction on a given table with the nodejs driver, which may take 30 minutes.
While doing that, if I try to query with select on that table using the jdbc driver, the query execution waits for the transaction to finish. If I close the transaction (by finishing it or by forcing it to exit), the jdbc query becomes responsive.
I thought I can read a table with one connection while performing a transaction with another one.
What am I missing here?
Should I keep the table (without dropping it at the beginning of the transaction) ?
DROP TABLE takes an ACCESS EXCLUSIVE lock on the table, which is there precisely to prevent it from taking place concurrently with any other operation on the table. After all, DROP TABLE physically removes the table.
Since all locks are held until the end of the database transaction, all access to the dropped table is blocked until the transaction ends.
Of course the files are only removed when the transaction commits, so you might wonder why PostgreSQL doesn't let concurrent transactions read in the mean time. But that would mean that COMMIT may be blocked by a concurrent reader, or a SELECT might cause a system error in the middle of reading, both of which don't sound appealing.
I'm migrating data from one table to another in an environment where any long locks or downtime is not acceptable, in total about 80000 rows. Essentially the query boils down to this simple case:
INSERT INTO table_2
SELECT * FROM table_1
JOIN table_3 on table_1.id = table_3.id
All 3 tables are being read from and could have an insert at any time. I want to just run the query above, but I'm not sure how the locking works and whether the tables will be totally inaccessible during the operation. My understanding tells me that only the affected rows (newly inserted) will be locked. Table 1 is just being selected, so no harm, and concurrent inserts are safe so table 2 should be freely accessible.
Is this understanding correct, and can I run this query in a production environment without fear? If it's not safe, what is the standard way to accomplish this?
You're fine.
If you're interested in the details, you can read up on multiversion concurrency control, or on the details of the Postgres MVCC implementation, or how its various locking modes interact, but the implications for your case are nicely summarised in the docs:
reading never blocks writing and writing never blocks reading
In short, every record stored in the database has some version number attached to it, and every query knows which versions to consider and which to ignore.
This means that an INSERT can safely write to a table without locking it, as any concurrent queries will simply ignore the new rows until the inserting transaction decides to commit.
How do I acquire a row-specific lock in Postgres?
According to the documentation, I can acquire a table lock, but I don't want to lock the whole table; I only care about monitoring a specific row in a given transaction.
My use case is that I want to:
read a row
perform some (potentially expensive, potentially race-condition) operations that prepare new rows for another table
when I'm finally ready to insert/update these new rows, I want to make sure no other process beat me to the punch, so I think I want to:
acquire a row-level lock on the same row I read earlier
see if the version or state or column like that is the same as when I read it earlier
if it is the same, I want to insert/update into the other table, then increment this version, then release the lock
if it is not the same version, then I want to abort - I don't care about my possible new/updated rows, they are stale and I don't want to save them (so: do nothing, release the lock)
But, I don't want to lock the entire table for this whole period; I just want to lock that one specific row. I cannot figure out how to do that based on numerous . blog . posts and Postgres . documentation
All I want is an example query that shows me how to explicitly row-level lock.
I like to do batch updates to Postgres. Sometimes, the batch may contain update-statements to the same record. (*)
To this end I need to be sure that Postgres locks rows based on the order in which the update-statements are supplied.
Is this guaranteed?
To be clear, I'm sending a sequence of single row update-statements, so not a single multi-row update-statement. E.g.:
update A set x='abc', dt='<timeN>' where id='123';
update A set x='def', dt='<timeN+1>' where id='123';
update A set x='ghi', dt='<timeN+2>' where id='123';
*) This might seem redundant: just only save the last one. However, I have defined an after-trigger on the table so history is created in a different table. Therefore I need the multiple updates.
The rows will definitely be locked in the order of the UPDATE statements.
Moreover, locks only affect concurrent transactions, so if all the UPDATEs take place in one database session, you don't have to be afraid to get blocked by a lock.
Postgres 9.4, Ubuntu 10
I have been unable to find this exact problem here, so here it goes:
For each table t in my database, I have a table t_audit. Each delete, insert, and update on table t triggers a function that inserts a record to table t_audit.
Each night, a process truncates each t_audit table.
Last night, a select on table t prevented the truncate on t_audit from proceeding. I did not save what was in pg_stat_activity at the time, but I did save the output from blocking_locks().
Blocking pid: RowExclusiveLock, t, select * from t where ...,
Waiting pid: AccessExclusiveLock, t_audit, truncate table t_audit,
I am uncertain as to why a select on t would block the truncate on t_audit. As I did not save pg_stat_activity, the best that I can assume is that the select was "idle in transaction". I asked the person who was running the query at the time, and he said he was not running the update as part of a transaction. He did update table t just prior to the select. He did not close his connection as the pid was still active until I ran pg_terminate_backend on the pid.
Has anyone experienced this issue before? Is there a recommended procedure for this other than running pg_terminate_backend on any pids which are "idle in transaction" just prior to calling truncates?
Thank you for reading and taking time to respond.
Are there any triggers in place that might cause even something as innocuous as a SELECT on the audit table at the same time as the TRUNCATE (although the fact that it's a Row Exclusive lock indicates that whatever is being triggered is something like an UPDATE instead)? From the PG 9.4 locking documentation, SELECT and TRUNCATE would indeed block each other as expected behavior. The relevant tidbits are these:
ACCESS SHARE
Conflicts with the ACCESS EXCLUSIVE lock mode only.
The SELECT command acquires a lock of this mode on referenced tables. In general, any query that only reads a table and does not modify it will acquire this lock mode.
ACCESS EXCLUSIVE
Conflicts with locks of all modes (ACCESS SHARE, ROW SHARE, ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE, and ACCESS EXCLUSIVE). This mode guarantees that the holder is the only transaction accessing the table in any way.
Acquired by the DROP TABLE, TRUNCATE, REINDEX, CLUSTER, and VACUUM FULL commands. Many forms of ALTER TABLE also acquire a lock at this level.
And even more specifically telling is this explicit tip on that page:
Tip: Only an ACCESS EXCLUSIVE lock blocks a SELECT (without FOR UPDATE/SHARE) statement.
As for what to do in this scenario, if your use case is tolerant of unceremonious terminations of (possibly idle) connections, that is certainly a straightforward way of ensuring that the TRUNCATE succeeds.
A more flexible alternative may be to clear out the table with DELETE instead, and then follow up with some variation of VACUUM afterwards (DELETE and SELECT will not block each other, but it will block UPDATE). The suitability of this approach would depend a lot on things like the growth pattern of the table from day-to-day (simply VACUUM may be suitable if its maximum size is not that different day-to-day) and how badly you need that space reclaimed in the short term if it is a huge table - you may need to VACUUM FULL that table during a quiet window if you need the space quickly and badly, but VACUUM FULL is not a gentle hammer to swing by any means.