Oracle updating large tables
A huge DML/DDL activity would take lot of time and would result in space usage problems and heavy resource utilization, hence such batch processing is normally scheduled in off peak hours.There are many ways of creating a column with a default value or updating an existing column.Let’s say this is how STUDENT_N looks before it is merged with STUDENT: merge into student a 2 using 3 (select id, name, score 4 from student_n) b 5 on (= b.id) 6 when matched then 7 update set = 8 , a.score = b.score 9 when not matched then 10 insert (a.id, a.name, a.score) 11 values (b.id, b.name, b.score); 5 rows merged.select * from student; ID NAME SCORE ---------- --------------- ---------- 1 Jack 540 2 Rose 3 William 650 4 Caledon 620 5 Fabrizio 600 6 Thomas 7 Ruth 690 11 Brock 705 10 Lizzy 9 Wallace 600 8 Spicer 620 11 rows selected. But it does not work with a merge query (probably because of the update part): “Missing IN or OUT parameter at index:: 1” Would would you advise to retrieve the value of a field (typically an id) of the row that has been inserted or updated?
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select * from student; ID NAME SCORE ---------- --------------- ---------- 1 Jack 540 2 Rose 3 William 650 4 Caledon 620 5 Fabrizio 600 6 Thomas 7 Ruth 690 11 Brock 705 10 Lizzy 9 Wallace 600 8 Spicer 620 11 rows selected. Thanks, Hi egavaldo, MERGE doesn’t support the RETURNING clause and going by this Ask Tom thread, the feature isn’t likely to be available anytime soon.
SQL In the MERGE statement above, Jack, Caledon and Fabrizio’s records were not deleted from the table STUDENTS though their scores are below 640, since the delete condition works on the source, not the target. Interestingly, I have found it hard to get managers to agree to using it – they don’t like the sequence gaps and the fact that inserts and update counts aren’t reported individually. Hi, How do you know to know the id of the row that has been inserted OR updated? A workaround is to add a couple of tracking columns to the target table: transaction_type, transaction_date.
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If the statistics are out-of-date, the CBO might generate inappropriate access paths (such as using the wrong index), resulting in poor performance. If statistics are available for a table, the database system uses the cost-based optimizer. Since from Oracle 10g the rule-based optimizer is no longer supported, in this case auto-sampling is performed.