Here's what works already, but it's using a loop:
(I am updating the nickname, slug field on the remote table for each row in a local table)
DECLARE
row_ record;
rdbname_ varchar;
....
/* select from local */
FOR row_ IN SELECT rdbname, objectvalue1 as keyhash, cvalue1 as slug, cvalue2 as nickname
FROM bme_tag
where rdbname = rdbname_
and tagtype = 'NAME'
and wkseq = 0
LOOP
/* update remote */
PERFORM dblink_exec('sysdb',
format(
'update bme_usergroup
set nickname = %L
,slug = %L
where rdbname = %L
and wkseq = 0
and keyhash = %L'
, row_.nickname, row_.slug, row_.rdbname, row_.keyhash)
);
END LOOP;
Now, what I would like to do instead is to do a bulk UPDATE (remote) FROM (local)
PERFORM dblink_exec('sysdb',
'update (remote)bme_usergroup
set nickname = bme_tag.cvalue2, slug=bme_tag.cvalue1
from (local).bme_tag s
where bme_usergroup.rdbname = %L
and bme_usergroup.wkseq = 0
and bme_usergroup.keyhash = s.keyhash
and bme_usergroup.rdbname = s.rdbname
)
I've gotten this far by looking a various solutions (postgresql: INSERT INTO ... (SELECT * ...)) and I know how to separate the remote and local tables of the query in the context of SELECT, DELETE and even INSERT/SELECT. And I can do that direct update with bind variables too. But how about UPDATE FROM?
If it's not possible, should I look into Postgres's FOREIGN TABLE or something similar?
The local and remote db are both on the same Postgres server. One additional bit of information, if it matters, is that either database may be dropped and restored separately from the other, and I'd prefer a lightweight solution that doesn't take a lot of configuration each time to reestablish communication.
Yes, you should use foreign tables with postgres_fdw.
That way you could just write your UPDATE statement like you would for a local table.
This should definitely be faster, but you might still be exchanging a lot of data between the databases.
If that's an option, it will probably be fastest to run the statement on the database where the updated table is and define the other table as a foreign table. That way you will probably avoid fetching and then sending the table data.
Use EXPLAIN to see what exactly happens!
Related
I am hoping that I can articulate this effectively, so here it goes:
I am creating a model which will be run on a platform by users, possibly simultaneously, but each model run is marked by a unique integer identifier. This model will execute a series of PostgreSQL queries and eventually write a result elswehere.
Now because of the required parallelization of model runs, I have to make sure that the processes will not collide, despite running in the same database. I am at a point now where I have to store a list of records, sorted by a score variable and then operate on them. This is the beginning of the query:
DO
$$
DECLARE row RECORD;
BEGIN
DROP TABLE IF EXISTS ranked_clusters;
CREATE TEMP TABLE ranked_clusters AS (
SELECT
pl.cluster_id AS c_id,
SUM(pl.total_area) AS cluster_score
FROM
emob.parking_lots AS pl
WHERE
pl.cluster_id IS NOT NULL
AND
run_id = 2005149
GROUP BY
pl.cluster_id
ORDER BY
cluster_score DESC
);
FOR row IN SELECT c_id FROM ranked_clusters LOOP
RAISE NOTICE 'Cluster %', row.c_id;
END LOOP;
END;
$$ LANGUAGE plpgsql;
So I create a temporary table called ranked_clusters and then iterate through it, at the moment just logging the identifiers of each record.
I have been careful to only build this list from records which have a run_id value equal to a certain number, so data from the same source, but with a different number will be ignored.
What I am worried about however is that a simultaneous process will also create its own ranked_clusters temporary table, which will collide with the first one, invalidating the results.
So my question is essentially this: Are temporary tables only visible to the session which creates them (or to the cursor object from say, Python)? And is it therefore safe to use a temporary table in this way?
The main reason I ask is because I see that these so-called "temporary" tables seem to persist after I execute the query in PgAdmin III, and the query fails on the next execution because the table already exists. This troubles me because it seems as though the tables are actually globally accessible during their lifetime and would therefore introduce the possibility of a collision when a simultaneous run occurs.
Thanks #a_horse_with_no_name for the explanation but I am not yet convinced that it is safe, because I have been able to execute the following code:
import psycopg2 as pg2
conn = pg2.connect(dbname=CONFIG["GEODB_NAME"],
user=CONFIG["GEODB_USER"],
password=CONFIG["GEODB_PASS"],
host=CONFIG["GEODB_HOST"],
port=CONFIG["GEODB_PORT"])
conn.autocommit = True
cur = conn.cursor()
conn2 = pg2.connect(dbname=CONFIG["GEODB_NAME"],
user=CONFIG["GEODB_USER"],
password=CONFIG["GEODB_PASS"],
host=CONFIG["GEODB_HOST"],
port=CONFIG["GEODB_PORT"])
conn2.autocommit = True
cur2 = conn.cursor()
cur.execute("CREATE TEMPORARY TABLE temptable (tempcol INTEGER); INSERT INTO temptable VALUES (0);")
cur2.execute("SELECT tempcol FROM temptable;")
print(cur2.fetchall())
And I receive the value in temptable despite it being created as a temporary table in a completely different connection as the one which queries it afterwards. Am I missing something here? Because it seems like the temporary table is indeed accessible between connections.
The above had a typo, Both cursors were actually being spawned from conn, rather than one from conn and another from conn2. Individual connections in psycopg2 are not able to access each other's temporary tables, but cursors spawned from the same connection are.
Temporary tables are only visible to the session (=connection) that created them. Even if two sessions create the same table, they won't interfere with each other.
Temporary tables are removed automatically when the session is disconnected.
If you want to automatically remove them when your transaction ends, use the ON COMMIT DROP option when creating the table.
So the answer is: yes, this is safe.
Unrelated, but: you can't store rows "in a sorted way". Rows in a table have no implicit sort order. The only way you can get a guaranteed sort order is to use an ORDER BY when selecting the rows. The order by that is part of your CREATE TABLE AS statement is pretty much useless.
If you have to rely on the sort order of the rows, the only safe way to do that is in the SELECT statement:
FOR row IN SELECT c_id FROM ranked_clusters ORDER BY cluster_score
LOOP
RAISE NOTICE 'Cluster %', row.c_id;
END LOOP;
I have googled quite a bit, and I have fairly decent reading comprehension, but I don't understand if this script will work in multiple threads on my postgres/postgis box. Here is the code:
Do
$do$
DECLARE
x RECORD;
b int;
begin
create temp table geoms (id serial, geom geometry) on commit drop;
for x in select id,geom from asdf loop
truncate table geoms;
insert into geoms (geom) select someGeomfield from sometable where st_intersects(somegeomfield,x.geom);
----do something with the records in geoms here...and insert that data somewhere else
end loop;
end;
$do$
So, if I run this in more than one client, called from Java, will the scope of the geoms temp table cause problems? If so, any ideas for a solution to this in PostGres would be helpful.
Thanks
One subtle trap you will run into though, which is why I am not quite ready to declare it "safe" is that the scope is per session, but people often forget to drop the tables (so they drop on disconnect).
I think you are much better off if you don't need the temp table after your function to drop it explicitly after you are done with it. This will prevent issues that arise from trying to run the function twice in the same transaction. (On commit you are dropping)
Temp tables in PostgreSQL (or Postgres) (PostGres doesn't exists) are local only and related to session where they are created. So no other sessions (clients) can see temp tables from other session. Both (schema and data) are invisible for others. Your code is safe.
In my script I have to do a lot of selects to a joined table, so instead I decided to put this join into a temporal table.
First I thought:
1. Create table
2. Put the data from the join into a table
3. Drop the table
But then I thought, what if the script fails before I dropped the table?
So I decided to go with:
1. Drop the table
2. Create the table
3. Put the data from the join into a table
I don't really mind if the table is left there until the next time I run the script, so the second option works too.
But what if somebody had already dropped the table?
I saw some systems have a "drop if exists" but unfortunately not DB2. I would like to do something that won't make the script die when the drop table fails.
Ideas? On any of this? Thanks!
EDIT: I forgot to say this is in a PERL script!
The best way to do this is by using an annonymous block like in this code
You need to call the drop table in a dynamic sql, and catch the exception in the block.
--#SET TERMINATOR #
begin
declare statement varchar(128);
declare continue handle for sqlstate '42710' BEGIN END;
SET STATEMENT = 'DROP TABLE MYTABLE';
EXECUTE IMMEDIATE STATEMENT;
end #
This code will run normally in DB2. It does not need to be part of a procedure nor function.
Why not look for the table first? If you find it, it needs to be dropped; if you don't, it doesn't.
db2perf_quiet_drop that might works the way you want.. Its a free add-on :)
You can look into this post too..
http://www.dbforums.com/showthread.php?1609047-DB2-equivalent-for-mysql-s-DROP-TABLE-IF-EXISTS
If this doesn't work for you please let me know what error you are getting so I can try to help :)
Or this might work
if( NOT exists( create table detailval
(
id int,
detaildeptNo int,
info varchar(255)
);
insert into detailval(1,1, 'detail values A');
insert into detailval(2,1, 'detail values B');
insert into detailval(3,1, 'detail values C');
insert into detailval(4,2, 'detail values D');
)
)
then customStoredproc('droptable');
end if;
End
I think you should look into working with temporary tables (DECLARE GLOBAL TEMPORARY TABLE). They are stored in the temporary table space and are dropped automatically after commit.
You can easily also query syscat.tables like this:
select COUNT(*) from SYSCAT.TABLES where TRIM(TABNAME) = '<some_table_name>'
if this query returns 0 then the table does not exists.
I am building a multi tenant system in which many clients data will be in the same database.
I am paranoid about some developer forgetting to put the appropriate "WHERE clientid = " onto every query.
Is there a way to, at the database level, ensure that every query has the correct WHERE = clause, thereby ensuring that no query will ever be executed without also specifying which client the query is for?
I was wondering if maybe the query rewrite rules could do this but it's not clear to me if they can do so.
thanks
Deny permissions on the table t for all users. Then give them permission on a function f that returns the table and accepts the parameter client_id:
create or replace function f(_client_id integer)
returns setof t as
$$
select *
from t
where client_id = _client_id
$$ language sql
;
select * from f(1);
client_id | v
-----------+---
1 | 2
Another way is to create a VIEW for:
SELECT *
FROM t
WHERE t.client_id = current_setting('session_vars.client_id');
And use SET session_vars.client_id = 1234 at the start of the session.
Deny acces to the tables, and leave only permissins for views.
You may need to create rewrite rules for UPDATE, DELETE, INSERT for the views (it depends on your PostgreSQL version).
Performance penalty will be small (if any) because PostgreSQL will rewrite the queries before execution.
I want to do a large update on a table in PostgreSQL, but I don't need the transactional integrity to be maintained across the entire operation, because I know that the column I'm changing is not going to be written to or read during the update. I want to know if there is an easy way in the psql console to make these types of operations faster.
For example, let's say I have a table called "orders" with 35 million rows, and I want to do this:
UPDATE orders SET status = null;
To avoid being diverted to an offtopic discussion, let's assume that all the values of status for the 35 million columns are currently set to the same (non-null) value, thus rendering an index useless.
The problem with this statement is that it takes a very long time to go into effect (solely because of the locking), and all changed rows are locked until the entire update is complete. This update might take 5 hours, whereas something like
UPDATE orders SET status = null WHERE (order_id > 0 and order_id < 1000000);
might take 1 minute. Over 35 million rows, doing the above and breaking it into chunks of 35 would only take 35 minutes and save me 4 hours and 25 minutes.
I could break it down even further with a script (using pseudocode here):
for (i = 0 to 3500) {
db_operation ("UPDATE orders SET status = null
WHERE (order_id >" + (i*1000)"
+ " AND order_id <" + ((i+1)*1000) " + ")");
}
This operation might complete in only a few minutes, rather than 35.
So that comes down to what I'm really asking. I don't want to write a freaking script to break down operations every single time I want to do a big one-time update like this. Is there a way to accomplish what I want entirely within SQL?
Column / Row
... I don't need the transactional integrity to be maintained across
the entire operation, because I know that the column I'm changing is
not going to be written to or read during the update.
Any UPDATE in PostgreSQL's MVCC model writes a new version of the whole row. If concurrent transactions change any column of the same row, time-consuming concurrency issues arise. Details in the manual. Knowing the same column won't be touched by concurrent transactions avoids some possible complications, but not others.
Index
To avoid being diverted to an offtopic discussion, let's assume that
all the values of status for the 35 million columns are currently set
to the same (non-null) value, thus rendering an index useless.
When updating the whole table (or major parts of it) Postgres never uses an index. A sequential scan is faster when all or most rows have to be read. On the contrary: Index maintenance means additional cost for the UPDATE.
Performance
For example, let's say I have a table called "orders" with 35 million
rows, and I want to do this:
UPDATE orders SET status = null;
I understand you are aiming for a more general solution (see below). But to address the actual question asked: This can be dealt with in a matter milliseconds, regardless of table size:
ALTER TABLE orders DROP column status
, ADD column status text;
The manual (up to Postgres 10):
When a column is added with ADD COLUMN, all existing rows in the table
are initialized with the column's default value (NULL if no DEFAULT
clause is specified). If there is no DEFAULT clause, this is merely a metadata change [...]
The manual (since Postgres 11):
When a column is added with ADD COLUMN and a non-volatile DEFAULT
is specified, the default is evaluated at the time of the statement
and the result stored in the table's metadata. That value will be used
for the column for all existing rows. If no DEFAULT is specified,
NULL is used. In neither case is a rewrite of the table required.
Adding a column with a volatile DEFAULT or changing the type of an
existing column will require the entire table and its indexes to be
rewritten. [...]
And:
The DROP COLUMN form does not physically remove the column, but
simply makes it invisible to SQL operations. Subsequent insert and
update operations in the table will store a null value for the column.
Thus, dropping a column is quick but it will not immediately reduce
the on-disk size of your table, as the space occupied by the dropped
column is not reclaimed. The space will be reclaimed over time as
existing rows are updated.
Make sure you don't have objects depending on the column (foreign key constraints, indices, views, ...). You would need to drop / recreate those. Barring that, tiny operations on the system catalog table pg_attribute do the job. Requires an exclusive lock on the table which may be a problem for heavy concurrent load. (Like Buurman emphasizes in his comment.) Baring that, the operation is a matter of milliseconds.
If you have a column default you want to keep, add it back in a separate command. Doing it in the same command applies it to all rows immediately. See:
Add new column without table lock?
To actually apply the default, consider doing it in batches:
Does PostgreSQL optimize adding columns with non-NULL DEFAULTs?
General solution
dblink has been mentioned in another answer. It allows access to "remote" Postgres databases in implicit separate connections. The "remote" database can be the current one, thereby achieving "autonomous transactions": what the function writes in the "remote" db is committed and can't be rolled back.
This allows to run a single function that updates a big table in smaller parts and each part is committed separately. Avoids building up transaction overhead for very big numbers of rows and, more importantly, releases locks after each part. This allows concurrent operations to proceed without much delay and makes deadlocks less likely.
If you don't have concurrent access, this is hardly useful - except to avoid ROLLBACK after an exception. Also consider SAVEPOINT for that case.
Disclaimer
First of all, lots of small transactions are actually more expensive. This only makes sense for big tables. The sweet spot depends on many factors.
If you are not sure what you are doing: a single transaction is the safe method. For this to work properly, concurrent operations on the table have to play along. For instance: concurrent writes can move a row to a partition that's supposedly already processed. Or concurrent reads can see inconsistent intermediary states. You have been warned.
Step-by-step instructions
The additional module dblink needs to be installed first:
How to use (install) dblink in PostgreSQL?
Setting up the connection with dblink very much depends on the setup of your DB cluster and security policies in place. It can be tricky. Related later answer with more how to connect with dblink:
Persistent inserts in a UDF even if the function aborts
Create a FOREIGN SERVER and a USER MAPPING as instructed there to simplify and streamline the connection (unless you have one already).
Assuming a serial PRIMARY KEY with or without some gaps.
CREATE OR REPLACE FUNCTION f_update_in_steps()
RETURNS void AS
$func$
DECLARE
_step int; -- size of step
_cur int; -- current ID (starting with minimum)
_max int; -- maximum ID
BEGIN
SELECT INTO _cur, _max min(order_id), max(order_id) FROM orders;
-- 100 slices (steps) hard coded
_step := ((_max - _cur) / 100) + 1; -- rounded, possibly a bit too small
-- +1 to avoid endless loop for 0
PERFORM dblink_connect('myserver'); -- your foreign server as instructed above
FOR i IN 0..200 LOOP -- 200 >> 100 to make sure we exceed _max
PERFORM dblink_exec(
$$UPDATE public.orders
SET status = 'foo'
WHERE order_id >= $$ || _cur || $$
AND order_id < $$ || _cur + _step || $$
AND status IS DISTINCT FROM 'foo'$$); -- avoid empty update
_cur := _cur + _step;
EXIT WHEN _cur > _max; -- stop when done (never loop till 200)
END LOOP;
PERFORM dblink_disconnect();
END
$func$ LANGUAGE plpgsql;
Call:
SELECT f_update_in_steps();
You can parameterize any part according to your needs: the table name, column name, value, ... just be sure to sanitize identifiers to avoid SQL injection:
Table name as a PostgreSQL function parameter
Avoid empty UPDATEs:
How do I (or can I) SELECT DISTINCT on multiple columns?
Postgres uses MVCC (multi-version concurrency control), thus avoiding any locking if you are the only writer; any number of concurrent readers can work on the table, and there won't be any locking.
So if it really takes 5h, it must be for a different reason (e.g. that you do have concurrent writes, contrary to your claim that you don't).
You should delegate this column to another table like this:
create table order_status (
order_id int not null references orders(order_id) primary key,
status int not null
);
Then your operation of setting status=NULL will be instant:
truncate order_status;
First of all - are you sure that you need to update all rows?
Perhaps some of the rows already have status NULL?
If so, then:
UPDATE orders SET status = null WHERE status is not null;
As for partitioning the change - that's not possible in pure sql. All updates are in single transaction.
One possible way to do it in "pure sql" would be to install dblink, connect to the same database using dblink, and then issue a lot of updates over dblink, but it seems like overkill for such a simple task.
Usually just adding proper where solves the problem. If it doesn't - just partition it manually. Writing a script is too much - you can usually make it in a simple one-liner:
perl -e '
for (my $i = 0; $i <= 3500000; $i += 1000) {
printf "UPDATE orders SET status = null WHERE status is not null
and order_id between %u and %u;\n",
$i, $i+999
}
'
I wrapped lines here for readability, generally it's a single line. Output of above command can be fed to psql directly:
perl -e '...' | psql -U ... -d ...
Or first to file and then to psql (in case you'd need the file later on):
perl -e '...' > updates.partitioned.sql
psql -U ... -d ... -f updates.partitioned.sql
I am by no means a DBA, but a database design where you'd frequently have to update 35 million rows might have… issues.
A simple WHERE status IS NOT NULL might speed up things quite a bit (provided you have an index on status) – not knowing the actual use case, I'm assuming if this is run frequently, a great part of the 35 million rows might already have a null status.
However, you can make loops within the query via the LOOP statement. I'll just cook up a small example:
CREATE OR REPLACE FUNCTION nullstatus(count INTEGER) RETURNS integer AS $$
DECLARE
i INTEGER := 0;
BEGIN
FOR i IN 0..(count/1000 + 1) LOOP
UPDATE orders SET status = null WHERE (order_id > (i*1000) and order_id <((i+1)*1000));
RAISE NOTICE 'Count: % and i: %', count,i;
END LOOP;
RETURN 1;
END;
$$ LANGUAGE plpgsql;
It can then be run by doing something akin to:
SELECT nullstatus(35000000);
You might want to select the row count, but beware that the exact row count can take a lot of time. The PostgreSQL wiki has an article about slow counting and how to avoid it.
Also, the RAISE NOTICE part is just there to keep track on how far along the script is. If you're not monitoring the notices, or do not care, it would be better to leave it out.
Are you sure this is because of locking? I don't think so and there's many other possible reasons. To find out you can always try to do just the locking. Try this:
BEGIN;
SELECT NOW();
SELECT * FROM order FOR UPDATE;
SELECT NOW();
ROLLBACK;
To understand what's really happening you should run an EXPLAIN first (EXPLAIN UPDATE orders SET status...) and/or EXPLAIN ANALYZE. Maybe you'll find out that you don't have enough memory to do the UPDATE efficiently. If so, SET work_mem TO 'xxxMB'; might be a simple solution.
Also, tail the PostgreSQL log to see if some performance related problems occurs.
I would use CTAS:
begin;
create table T as select col1, col2, ..., <new value>, colN from orders;
drop table orders;
alter table T rename to orders;
commit;
Some options that haven't been mentioned:
Use the new table trick. Probably what you'd have to do in your case is write some triggers to handle it so that changes to the original table also go propagated to your table copy, something like that... (percona is an example of something that does it the trigger way). Another option might be the "create a new column then replace the old one with it" trick, to avoid locks (unclear if helps with speed).
Possibly calculate the max ID, then generate "all the queries you need" and pass them in as a single query like update X set Y = NULL where ID < 10000 and ID >= 0; update X set Y = NULL where ID < 20000 and ID > 10000; ... then it might not do as much locking, and still be all SQL, though you do have extra logic up front to do it :(
PostgreSQL version 11 handles this for you automatically with the Fast ALTER TABLE ADD COLUMN with a non-NULL default feature. Please do upgrade to version 11 if possible.
An explanation is provided in this blog post.