Re: UPDATE many records

[Israel Brewster <ijbrewster@xxxxxxxxxx>]

One potential issue I just thought of with this approach: disk space. Will I be doubling the amount of space used while both tables exist? If so, that would prevent this from working - I don’t have that much space available at the moment.

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Israel Brewster
Software Engineer
Alaska Volcano Observatory 
Geophysical Institute - UAF 
2156 Koyukuk Drive 
Fairbanks AK 99775-7320

Work: 907-474-5172
cell:  907-328-9145

On Jan 7, 2020, at 10:09 AM, Mark Zellers <markz@xxxxxxxxxxxxxxxxxxxx> wrote:

You don’t tell us if other users will be concurrently changing any of the records involved.  If you could guarantee that the table won’t be changed, you might be better off doing a CREATE TABLE table_new as SELECT … FROM table_old, dropping table_old, and finally renaming table_new.   Given the way Postgres handles updates, I would think that might perform significantly better.  Even if you did the work in batches (create a new table, insert/select from the old table, drop, rename), that could well be better.  Especially if you re-create the indexes after all the data is moved.

 

 

 

From: Israel Brewster <ijbrewster@xxxxxxxxxx> 
Sent: Monday, January 6, 2020 10:36 AM
To: pgsql-general@xxxxxxxxxxxxxxxxxxxx
Subject: UPDATE many records

 

Thanks to a change in historical data, I have a need to update a large number of records (around 50 million). The update itself is straight forward, as I can just issue an "UPDATE table_name SET changed_field=new_value();" (yes, new_value is the result of a stored procedure, if that makes a difference) command via psql, and it should work. However, due to the large number of records this command will obviously take a while, and if anything goes wrong during the update (one bad value in row 45 million, lost connection, etc), all the work that has been done already will be lost due to the transactional nature of such commands (unless I am missing something).

 

Given that each row update is completely independent of any other row, I have the following questions:

 

1) Is there any way to set the command such that each row change is committed as it is calculated?

2) Is there some way to run this command in parallel in order to better utilize multiple processor cores, other than manually breaking the data into chunks and running a separate psql/update process for each chunk? Honestly, manual parallelizing wouldn’t be too bad (there are a number of logical segregations I can apply), I’m just wondering if there is a more automatic option.

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Israel Brewster
Software Engineer
Alaska Volcano Observatory 
Geophysical Institute - UAF 
2156 Koyukuk Drive 
Fairbanks AK 99775-7320

Work: 907-474-5172
cell:  907-328-9145