Inserting multiple rows into Oracle

Question

In the discussion about multiple row insert into the Oracle two approaches were demonstrated:

First:

insert into pager (PAG_ID,PAG_PARENT,PAG_NAME,PAG_ACTIVE)
          select 8000,0,'Multi 8000',1 from dual
union all select 8001,0,'Multi 8001',1 from dual

Second:

INSERT ALL
   INTO t (col1, col2, col3) VALUES ('val1_1', 'val1_2', 'val1_3')
   INTO t (col1, col2, col3) VALUES ('val2_1', 'val2_2', 'val2_3')
   INTO t (col1, col2, col3) VALUES ('val3_1', 'val3_2', 'val3_3')
   .
   .
   .
SELECT 1 FROM DUAL;

Could anyone argue the preference of using one over another?

P.S. I didn't do any research myself (even explanation plan), so any information or opinion would be appreciated.

Thanks.

Answer 1

The statement utilizing the UNION ALL has theoretically a small performance disadvantage as it has to union the results of all statements before the insert can happen. The INSERT ALL doesn't have this disadvantage as the final result can already be processed line-by-line.

But practically the optimizer inside Oracle should make the difference negligible and it is up to your preferences which way you choose.

In my own opinion the INSERT ALL is the better human-readable of the two while the UNION ALL variant is the one taking less space when such an insert is automatically generated.

Answer 2

I would suspect solution 1 is a bit of a hack that works and is probably less efficient than the designed alternative of Insert ALL.

Insert all is really designed for you to insert many rows into more than 1 table as a result of a select, eg:

Insert ALL
into 
  t1 (c1, c2) values (q1, q2)
  t2 (x1, x2) values (q1, q3)
select q1, q2, q3 from t3

If you want to load thousands of rows and they are not in the database already, I don't think this is the best way to do it - If your data is in a file, you want to look at External Tables or SQL Loader to efficiently insert the rows for you.

Answer 3

From performance's point of view, these queries are identical.

UNION ALL won't hurt performance, since Oracle estimates the UNION'ed query only when it needs it, it doesn't cache the results first.

SELECT syntax is more flexible in that sense that you can more easuly manupulate the SELECT query if you want to change something.

For instance, this query:

insert into pager (PAG_ID,PAG_PARENT,PAG_NAME,PAG_ACTIVE)
          select 8000,0,'Multi 8000',1 from dual
union all select 8001,0,'Multi 8001',1 from dual

can be rewritten as

INSERT
INTO    pager (PAG_ID,PAG_PARENT,PAG_NAME,PAG_ACTIVE)
SELECT  7999 + level, 0, 'Multi ' || 7999 + level, 1
FROM    dual
CONNECT BY
        level <= 2

By replacing 2 with appropriate number, you can get any number of rows you want.

In case of INSERT ALL, you would have to duplicate the destination table description, which is less readable if you need, say, 40 rows.

Answer 4

Thanks everyone for participating and spending your time here. I really appreciate that, your answers were quite useful for me.