There is one table in my schema that does not get autovacuumed. If I run VACUUM posts; on the table the vacuum process finishes nicely, but the autovacuum daemon never vacuums the table for some reason.
Is there a way to find out why? What could be possible reasons for this?
That is just fine, nothing to worry.
The table is the only medium sized one (3 million rows).
Autovacuum will kick in if the number of dead tuples more than autovacuum_vacuum_scale_factor (default: 0.2) of your live tuples, so if more than 20% of your table has been deleted or updated.
This is usually just fine, and I would not change it. But if you want to do it for some reason, you can do it like this:
ALTER TABLE posts SET (autovacuum_vacuum_scale_factor = 0.1);
Related
I have a situation where updates on my temp table is slow. Below is the scenario
Created temp table in session for every session,first time temp table created and then going forward doing insert,update and delete operations this operations until session ends only.
First i'm inserting the rows and based on rows i'm updateing other columns. but this updates is slow compared to norma table. i checked the performance by replacing temp table whereas normal table taking around 50 to 60s but temp table is taking nearly 5 mins.
I tried analyze on temp table, then i got the improved performance. when im using analyze the updates are completed in with 50 seconds.
I tried Types also, but no luck.
Record count in temp table is 480
Can anyone help to imprrove the performance on temp table with out analyze OR any alternative for bulk collect and bulk insert in user defined types
All the above ooperations i'm doing in postgresql.
The lack of information in your question forces me to guess, but if all other things are equal, the difference is probably that you don't have accurate statistics on the temporary table. For normal tables (which are visible to the public), autovacuum takes care of that automatically, but for temporary tables, you have to call ANALYZE explicitly to gather table statistics.
We use PostgreSQL for analytics. Three typical operations we do on tables are:
Create table as select
Create table followed by insert in table
Drop table
We are not doing any UPDATE, DELETE etc.
For this situation can we assume that estimates would just be accurate?
SELECT reltuples AS estimate FROM pg_class where relname = 'mytable';
With autovacuum running (which is the default), ANALYZE and VACUUM are fired up automatically - both of which update reltuples. Basic configuration parameters for ANALYZE (which typically runs more often), (quoting the manual):
autovacuum_analyze_threshold (integer)
Specifies the minimum number of inserted, updated or deleted tuples
needed to trigger an ANALYZE in any one table. The default is 50
tuples. This parameter can only be set in the postgresql.conf file
or on the server command line; but the setting can be overridden for
individual tables by changing table storage parameters.
autovacuum_analyze_scale_factor (floating point)
Specifies a fraction of the table size to add to
autovacuum_analyze_threshold when deciding whether to trigger an
ANALYZE. The default is 0.1 (10% of table size). This parameter can
only be set in the postgresql.conf file or on the server command
line; but the setting can be overridden for individual tables by
changing table storage parameters.
Another quote gives insight to details:
For efficiency reasons, reltuples and relpages are not updated
on-the-fly, and so they usually contain somewhat out-of-date values.
They are updated by VACUUM, ANALYZE, and a few DDL commands such
as CREATE INDEX. A VACUUM or ANALYZE operation that does not
scan the entire table (which is commonly the case) will incrementally
update the reltuples count on the basis of the part of the table it
did scan, resulting in an approximate value. In any case, the planner
will scale the values it finds in pg_class to match the current
physical table size, thus obtaining a closer approximation.
Estimates are up to date accordingly. You can change autovacuum settings to be more aggressive. You can even do this per table. See:
Aggressive Autovacuum on PostgreSQL
On top of that, you can scale estimates like Postgres itself does it. See:
Fast way to discover the row count of a table in PostgreSQL
Note that VACUUM (of secondary relevance to your case) wasn't triggered by only INSERTs before Postgres 13. Quoting the release notes:
Allow inserts, not only updates and deletes, to trigger vacuuming
activity in autovacuum (Laurenz Albe, Darafei
Praliaskouski)
Previously, insert-only activity would trigger auto-analyze but not
auto-vacuum, on the grounds that there could not be any dead tuples to
remove. However, a vacuum scan has other useful side-effects such as
setting page-all-visible bits, which improves the efficiency of
index-only scans. Also, allowing an insert-only table to receive
periodic vacuuming helps to spread out the work of “freezing” old
tuples, so that there is not suddenly a large amount of freezing work
to do when the entire table reaches the anti-wraparound threshold all
at once.
If necessary, this behavior can be adjusted with the new parameters
autovacuum_vacuum_insert_threshold and
autovacuum_vacuum_insert_scale_factor, or the equivalent
table storage options.
I know the statistical information is updated by VACUUM ANALYZE and CREATE INDEX, but I'm not sure about some other situations:
insert new data into a table
let the database do nothing (and wait for autovacuum?)
delete some rows in a table
truncate a partition of a table
CREATE INDEX does not cause new statistics to be calculated.
The autovacuum daemon will run an ANALYZE process for all tables that have more than 10% of their data changed (this is the default configuration). Theres changes are INSERT, UPDATE or DELETE. TRUNCATE will clear the statistics for a table.
I'm retrieving data from an AWS database using PgAdmin. This works well. The problem is that I have one column that I set to True after I retrieve the corresponding row, where originally it is set to Null. Doing so adds an enormous amount of data to my database.
I have checked that this is not due to other processes: it only happens when my program is running.
I am certain no rows are being added, I have checked the number of rows before and after and they're the same.
Furthermore, it only does this when changing specific tables, when I update other tables in the same database with the same process, the database size stays the same. It also does not always increase the database size, only once every couple changes does the total size increase.
How can changing a single boolean from Null to True add 0.1 MB to my database?
I'm using the following commands to check my database makeup:
To get table sizes
SELECT
relname as Table,
pg_total_relation_size(relid) As Size,
pg_size_pretty(pg_total_relation_size(relid) - pg_relation_size(relid)) as External Size
FROM pg_catalog.pg_statio_user_tables ORDER BY pg_total_relation_size(relid) DESC;
To get number of rows:
SELECT schemaname,relname,n_live_tup
FROM pg_stat_user_tables
ORDER BY n_live_tup DESC;
To get database size:
SELECT pg_database_size('mydatabasename')
If you have not changed that then your fillfactor is at 100% on the table since that is the default.
This means that every change in your table will mark the changed row as obsolete and will recreate the updated row. The issue could be even worse if you have indices on your table since those should be updated on every row change too. As you could imagine this hurts the UPDATE performance too.
So technically if you would read the whole table and update even the smallest column after reading the rows then it would double the table size when your fillfactor is 100.
What you can do is to ALTER your table lower the fillfactor on it, then VACUUM it:
ALTER TABLE your_table SET (fillfactor = 90);
VACUUM FULL your_table;
Of course with this step your table will be about 10% bigger but Postgres will spare some space for your updates and it won't change its size with your process.
The reason why autovacuum helps is because it cleans the obsoleted rows periodically and therefore it will keep your table at the same size. But it puts a lot of pressure on your database. If you happen to know that you'll do operations like you described in the opening question then I would recommend tuning the fillfactor for your needs.
The problem is that (source):
"In normal PostgreSQL operation, tuples that are deleted or obsoleted by an update are not physically removed from their table"
Furthermore, we did not always close the cursor which also increased database size while running.
One last problem is that we were running one huge query, not allowing the system to autovacuum properly. This problem is described in more detail here
Our solution was to re-approach the problem such that the rows did not have to be updated. Other solutions that we could think of but have not tried is to stop the process every once in a while allowing the autovacuum to work correctly.
What do you mean adds data? to all the data files? specifically to some files?
to get a precise answer you should supply more details, but generally speaking, any DB operation will add data to the transaction logs, and possibly other files.
I'm doing massive tests on a Postgres database...
so basically I have 2 table where I inserted 40.000.000 records on, let's say table1 and 80.000.000 on table2
after this I deleted all those records.
Now if I do SELECT * FROM table1 it takes 199000ms ?
I can't understand what's happening?
can anyone help me on this?
If you delete all the rows from a table, they are marked as deleted but not actually removed from disk immediately. In order to remove them you need to do a "vacuum" operation- this should kick in automatically some time after such a big delete. Even so, that will just leave the pages empty but taking up quite a bit of disk space without a "vacuum full".
If you regularly need to do delete all the rows from a large table, consider using "truncate" instead, which simply zaps the table data file.
The tuples are logically deleted, not fisically.
You should perform a VACUUM on the db.
More info here
If you are deleting all records, use truncate not delete. Further the first time you run it the relation will not be cached (file cache or shared buffers), so it will be slower than subsequent times.