How can I tune my Oracle SQL query to run faster? [duplicate] - sql

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How to convert comma separated values to rows in oracle?
(6 answers)
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I have over 10,000 records in class_period table. When I run the query shown below, it takes too much time to fetch the data.
Can you please help me - how can I speed up the query?
WITH DATA AS
( SELECT distinct class_time , class_id
from class_period
)
SELECT distinct class_id, trim(regexp_substr(class_time, '[^:]+', 1, LEVEL)) class_time
FROM DATA
CONNECT BY regexp_substr(class_time , '[^:]+', 1, LEVEL) IS NOT NULL
sample data attached as image
enter image description here
required data attached as image
enter image description here
i am using oracle 11g.

Answer 1

Fix your query so you don't need to use DISTINCT. The problem with your method is that you are using a hierarchical query with multiple rows of input and no way of correlating each level of the hierarchy to the previous level so the query will correlate it to ALL the items at the previous level of the hierarchy and you will get exponentially more and more duplicate rows generated at each depth. This is incredibly inefficient.
Change from using regular expressions to simple string functions.
Instead you can use:
WITH bounds ( class_id, class_time, start_pos, end_pos ) AS (
SELECT class_id,
class_time,
1,
INSTR( class_time, ':', 1 )
FROM data
UNION ALL
SELECT class_id,
class_time,
end_pos + 1,
INSTR( class_time, ':', end_pos + 1 )
FROM bounds
WHERE end_pos > 0
)
SELECT class_id,
CASE end_pos
WHEN 0
THEN SUBSTR( class_time, start_pos )
ELSE SUBSTR( class_time, start_pos, end_pos - start_pos )
END AS class_time
FROM bounds;
Which, for the sample data:
CREATE TABLE data ( class_id, class_time ) AS
SELECT 1, '0800AM:0830AM' FROM DUAL UNION ALL
SELECT 1, '0900AM' FROM DUAL UNION ALL
SELECT 2, '0830AM:0900AM:0930AM' FROM DUAL UNION ALL
SELECT 2, '1000AM' FROM DUAL;
Outputs:
CLASS_ID | CLASS_TIME
-------: | :---------
1 | 0800AM
1 | 0900AM
2 | 0830AM
2 | 1000AM
1 | 0830AM
2 | 0900AM
2 | 0930AM
db<>fiddle here
However, an even better method would be to change your model for storing the data and stop storing it as a delimited string and instead store it in a separate table or, maybe, as a collection in a nested table.
An example using a second table is:
CREATE TABLE data (
class_id NUMBER PRIMARY KEY
);
CREATE TABLE class_times (
class_id NUMBER REFERENCES data ( class_id ),
class_time VARCHAR2(6)
);
INSERT ALL
INTO data ( class_id ) VALUES ( 1 )
INTO data ( class_id ) VALUES ( 2 )
INTO class_times ( class_id, class_time ) VALUES ( 1, '0800AM' )
INTO class_times ( class_id, class_time ) VALUES ( 1, '0830AM' )
INTO class_times ( class_id, class_time ) VALUES ( 1, '0900AM' )
INTO class_times ( class_id, class_time ) VALUES ( 2, '0830AM' )
INTO class_times ( class_id, class_time ) VALUES ( 2, '0900AM' )
INTO class_times ( class_id, class_time ) VALUES ( 2, '0930AM' )
INTO class_times ( class_id, class_time ) VALUES ( 2, '1000AM' )
SELECT * FROM DUAL;
Then your query would be (assuming that you need other columns from data alongside the class_id):
SELECT d.class_id,
c.class_time
FROM data d
INNER JOIN class_times c
ON ( d.class_id = c.class_id );
Which outputs:
CLASS_ID | CLASS_TIME
-------: | :---------
1 | 0800AM
1 | 0830AM
1 | 0900AM
2 | 0830AM
2 | 0900AM
2 | 0930AM
2 | 1000AM
An example using a nested table is:
CREATE TYPE stringlist IS TABLE OF VARCHAR2(6);
CREATE TABLE data (
class_id NUMBER,
class_time stringlist
) NESTED TABLE class_time STORE AS data__class_time;
INSERT INTO data ( class_id, class_time )
SELECT 1, stringlist( '0800AM','0830AM' ) FROM DUAL UNION ALL
SELECT 1, stringlist( '0900AM' ) FROM DUAL UNION ALL
SELECT 2, stringlist( '0830AM','0900AM','0930AM' ) FROM DUAL UNION ALL
SELECT 2, stringlist( '1000AM' ) FROM DUAL;
Then your query would become:
SELECT d.class_id,
ct.COLUMN_VALUE AS class_time
FROM data d
CROSS APPLY TABLE ( d.class_time ) ct
Which outputs:
CLASS_ID | CLASS_TIME
-------: | :---------
1 | 0800AM
1 | 0830AM
1 | 0900AM
2 | 0830AM
2 | 0900AM
2 | 0930AM
2 | 1000AM
db<>fiddle here

Answer 2

MT0 spotted the big problem of your connect by filter allowing all rows to be read. You don't need to convert it to a recursive CTE, since you're already distincting all the columns you're projecting, that can be treated as your primary key (assuming it's not nullable or you don't want the null values).
You also need a special filter so that it doesn't get confused into thinking you've got an infinite loop.
WITH DATA AS
( SELECT distinct class_time , class_id
from class_period
)
SELECT distinct class_id, trim(regexp_substr(class_time, '[^:]+', 1, LEVEL)) class_time
FROM DATA
CONNECT BY regexp_substr(class_time , '[^:]+', 1, LEVEL) IS NOT NULL
and prior class_time = class_time
and prior class_id = class_id
and prior sys_guid() is not null
The prior sys_guid() is not null is the special filter to prevent it from erroring with ORA-01436: CONNECT BY loop in user data.
This should perform similarly to the recursive CTE.