Say you've got the following query on 9i:
SELECT /*+ USE_HASH(t2 t3) */
* FROM
table1 t1 -- this has lots of rows
LEFT JOIN table2 t2 ON t1.col1 = t2.col1
AND t1.col2 = t2.col2
LEFT JOIN table3 t3 ON t1.col1 = t3.col1
AND t1.col2 = t3.col2
Due to 9i not having RIGHT OUTER HASH JOIN, it needs to hash table1 for both joins. Does it re-hash table1 between joining t2 and t3 (even though it's using the same join columns), or does it keep the same hash information for both joins?
It would need to rehash since the second hash would be table3 against the join of table1/table2 rather than against table1. Or vice versa.
For example, say TABLE1 had 100 rows, table2 had 50 and table3 had 10.
Joining table1 to table2 may give 500 rows. It then joins that result set to table3 to give (perhaps) 700 rows.
It won't do a join of table1 to table2, then a join of table1 to table3, then a join of those two intermediate results.
Look at the plan, it'll tell you the answer.
An example might be something like (I've just made this up):
SELECT
HASH JOIN
HASH JOIN
TABLE FULL SCAN table1
TABLE FULL SCAN table2
TABLE FULL SCAN table3
This sample plan involves a scan through table1, hashing its contents as it goes; scans through table2, hashes the results of the join into a second hash, then finally scans table3.
There are other plans it could choose from.
If table1 is the biggest table, and the optimizer knows this (due to stats), it probably won't drive from it though.
Related
Title was tough to choose my wording.
I have 2 tables I want to join together via a lg_code. Both columns are VARCHAR2(4 byte). I am running into an issue where table1 lg_code = 0003 and table2 lg_code = 3. The three 0's are causing an issue with the join and not returning all the data needed. How would I go about writing the join clause to fix this issue?
Code:
select * from table1 t1 JOIN table2 t2 ON t1.LG_CODE = t2.LG_CODE
I would suggest to convert the value of the columnlg_code to number first then make the join:
SELECT * FROM table1 t1
JOIN table2 t2 ON to_number(t1.LG_CODE) = to_number(t2.LG_CODE)
you can also use ltrim() on them:
SELECT * FROM table1 t1
JOIN table2 t2 ON LTRIM(t1.LG_CODE, '0') = LTRIM(t2.LG_CODE, '0');
but in newer versions of oracle SQL*PLUS it trims automatically.
Is there any difference between these two statements:
-- Statement 1:
SELECT *
FROM Table1 t1
LEFT OUTER JOIN TABLE2 t2 on t1.id = t2.id
and
-- Statement 2:
SELECT *
FROM Table1 t1
LEFT OUTER JOIN (SELECT id, a, b, c FROM Table2) t2 on t1.id = t2.id
I'm not an expert but statement 2 just looks like poorly written sql, and like it would take much longer. I'm attempting to optimize a code block and it has many joins like the second one. Are they technically the same and I can just replace with the standard join statement 1?
Thanks!
Ps. This is Oracle, and working with 100's of millions of rows.
PSS. I'm doing my own detective work to figure out if they are the same, and time differences, was hoping an expert could explain if there is a technical difference what it is.
They are not same queries, with the lack of a criteria in the subquery that depends on whether the all columns and all column names of the TABLE2 is involved in the subquery. If the subquery involves all of the column names of the TABLE2 in the select list then they are the same query and the subquery is unnecessary. With subquery I refer to the part with a select statement after the join statement in the parens.
The first one uses the TABLE2 with its all columns, all those columns will be available in the result set where the criteria met.
However in the second one the table you make the JOIN is not the TABLE2 of yours but a table with just columns from TABLE2 specified in the subquery's SELECT list, namely id, a, b, and c. But it will have all the rows after this subquery since no criteria is enforced on it by a WHERE clause in the subquery.
You will have same number of rows with only selected columns participating from the TABLE2.
The second one is not necessarily the poorly written one. You could have a criteria to met before you JOIN to the TABLE2.
I have a need to do the following
UPDATE TABLE2 t2
SET t2.product_id = (select t1.product_id from
table1 t1 where t1.matching_id = t2.matching_id)
Except that TABLE2 has 27 million records. The product_id is a newly added column and hence populating data to it.
I could use a cursor , break down my record set in TABLE2 to a reasonably smaller number, But with 27 million records, I am not sure whats the best way.
Pl suggest, even if it means exporting my data to excel.
Update - THe matching columns are indexed too.
The only thing I could do different is replace the update for a CREATE TABLE AS
CREATE TABLE table2_new AS
SELECT t2.* (less product_id), t1.product_id
FROM table1 t1
JOIN table2 t2
ON t1.matching_id = t2.matching_id
But later you will have to add the CONSTRAINTS manually, delete table2 and replace for table2_new
update (select t1.product_id as old_product_id, t2.product_id as new_product_id
from table1 t1
join table2 t2 on (t1.matching_id = t2.matching_id)) t
set t.new_product_id = t.old_product_id
UPDATE TABLE1 T1 SET T1.CENTERNAME=
(SELECT AC.CENTERNAME
FROM TABLE2 T2 INNER JOIN TABLET3 AN ON T2.CENTERID = T3.LOCATIONID
INNER JOIN TABLE1 T1 ON T3.LOG_ID = T1.LOGID W
HERE TRUNC(T1.ROW_DATE)='25-MAR-2014');
This gives the error 'ORA-01427: single-row subquery returns more than one row'.
The error message
ORA-01427: single-row subquery returns more than one row
means, er, the sub-query returns more than row. That is, this part of your statement ...
(SELECT AC.CENTERNAME
FROM TABLE2 T2 INNER JOIN TABLET3 AN ON T2.CENTERID = T3.LOCATIONID
INNER JOIN TABLE1 T1 ON T3.LOG_ID = T1.LOGID
WHERE TRUNC(T1.ROW_DATE)='25-MAR-2014')
returns more than row. The error occurs because the SET part of the UPDATE depends on the equality operator - SET T1.CENTERNAME= - so it can take only be one value.
Without more details about your data structure it is hard to be certain but I suspect what you really want is something like this
UPDATE TABLE1 T1
SET T1.CENTERNAME= (SELECT T2.CENTERNAME
FROM TABLE2 T2
INNER JOIN TABLE3 T3
ON T2.CENTERID = T3.LOCATIONID
WHERE T3.LOG_ID = T1.LOGID )
WHERE TRUNC(T1.ROW_DATE)='25-MAR-2014'
/
(I've tidied up your redaction to make the aliases consistent.)
I have two tables as: table1 with fields c1 and dt(nullable); table2 with fields start_dt, end_dt and wk_id. Now I need to perform left outer join between the table1 and table2 to take wk_id such that dt falls between start_dt and end_dt. I applied following condition but some wk_id which shouldn't be NULL are pulled NULL and some rows get repeated.
where nvl(t1.dt,'x') between nvl(t2.start_dt(+), 'x') and nvl(t2.end_dt(+), 'x');
What is wrong with the condition?
select *
from table1 t1
left join table2 t2
on t1.dt between t2.start_dt and t2.end_dt
I recommend you try the new ANSI join syntax.
Also, are you just using 'x' as an example? Or are the dt columns really stored as strings?
It seems you are missing the part "table1 left outer join table2 on table1.some_field = table2.some_field"
Something like this:
select t1.c1, t1.dt, t2.start_dt, t2.end_dt, t2.wk_id
from table1 t1 left outer join table2 t2
on t1.some_field1 = t2.some_field1
where nvl(t1.dt,'x')
between nvl(t2.start_dt, 'x') and
nvl(t2.end_dt, 'x')