On 4/6/24 13:04, yudhi s wrote:
On Sat, Apr 6, 2024 at 10:25 PM Adrian Klaver <adrian.klaver@xxxxxxxxxxx
<mailto:adrian.klaver@xxxxxxxxxxx>> wrote:
Your original problem description was:
"Then subsequently these rows will be inserted/updated based on the
delta number of rows that got inserted/updated in the source database.
In some cases these changed data can flow multiple times per day to the
downstream i.e. postgres database and in other cases once daily."
If the above is not a hard rule, then yes up to some point just
replacing the data in mass would be the simplest/fastest method. You
could cut a step out by doing something like TRUNCATE target_tab and
then COPY target_tab FROM 'source.csv' bypassing the INSERT INTO
source_tab.
Yes, actually i didn't realize that truncate table transactional/online
here in postgres. In other databases like Oracle its downtime for the
read queries on the target table, as data will be vanished from the
target table post truncate(until the data load happens) and those are
auto commit. Thanks Veem for sharing that option.
I also think that truncate will be faster if the changes/delta is
large , but if its handful of rows like <5%of the rows in the table then
Upsert/Merge will be better performant. And also the down side of the
truncate option is, it does ask to bring/export all the data from
source to the S3 file which may take longer as compared to bringing just
the delta records. Correct me if I'm wrong.
Since you still have not specified how the data is stored in S3 and how
you propose to move them into Postgres I can't really answer.
However I am still not able to understand why the upsert is less
performant than merge, could you throw some light on this please?
I have no idea how this works in the code, but my suspicion is it is due
to the following:
https://www.postgresql.org/docs/current/sql-insert.html#SQL-ON-CONFLICT
"The optional ON CONFLICT clause specifies an alternative action to
raising a unique violation or exclusion constraint violation error. For
each individual row proposed for insertion, either the insertion
proceeds, or, if an arbiter constraint or index specified by
conflict_target is violated, the alternative conflict_action is taken.
ON CONFLICT DO NOTHING simply avoids inserting a row as its alternative
action. ON CONFLICT DO UPDATE updates the existing row that conflicts
with the row proposed for insertion as its alternative action."
vs this:
"First, the MERGE command performs a join from data_source to
target_table_name producing zero or more candidate change rows. For each
candidate change row, the status of MATCHED or NOT MATCHED is set just
once, after which WHEN clauses are evaluated in the order specified. For
each candidate change row, the first clause to evaluate as true is
executed. No more than one WHEN clause is executed for any candidate
change row."
Where ON CONFLICT attempts the INSERT then on failure does the UPDATE
for the ON CONFLICT DO UPDATE case. MERGE on the hand evaluates based on
the join condition(ON tbl1.fld =tbl2.fld) and then based on MATCH/NOT
MATCHED takes the appropriate action for the first WHEN match. In other
words it goes directly to the appropriate action.
--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx
