How can we specify the order of the predicates execution?

Question

I am going to write the following query
select answer, answer_id from surveys s join answers a on s.survey_id = a.survey_seq_id
where question_uid = 206400374 and insertdate = to_date('31/12/2012') and answer > 0;However, when I look at the execution plan, I see that the last predicate (to_number(answer) > 0) has been executed the first. Henceforth, it checks many rows first. Normally, 75 rows belong to 31/12/2012 as you see from the following. Can I specify the execution order?
select count(*) from surveys s join answers a on s.survey_id = a.survey_seq_id
where question_uid = 206400374 and insertdate = to_date('31/12/2012');
COUNT(*)
75If so, how? Because, the type of answer is varchar2 and some of answers contain text characters such as 'Yes' or 'No'. Therefore, before 31/12/2012 some answers contain 'Yes' or 'No'. However in 31/12/2012 all answers are numeric.
Plan hash value: 3217836037
| Id  | Operation          | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |         |     2 |    68 |  9401   (1)| 00:01:53 |
|*  1 |  HASH JOIN         |         |     2 |    68 |  9401   (1)| 00:01:53 |
|*  2 |   TABLE ACCESS FULL| SURVEYS |     2 |    26 |   239   (1)| 00:00:03 |
|*  3 |   TABLE ACCESS FULL| ANSWERS |   337 |  7077 |  9162   (1)| 00:01:50 |
Predicate Information (identified by operation id):
   1 - access("S"."SURVEY_ID"="A"."SURVEY_SEQ_ID")
   2 - filter("S"."INSERTDATE"=TO_DATE(' 2012-12-31 00:00:00',
              'syyyy-mm-dd hh24:mi:ss'))
   3 - filter("A"."QUESTION_UID"=206400374 AND
              TO_NUMBER("A"."ANSWER")>0)
select distinct answer from surveys s join answers a on s.survey_id = a.survey_seq_id
where question_uid = 206400374 and insertdate = to_date('31/12/2012');
ANSWER
1
3
0
2And, also I can execute the following query without any error
select to_number(answer) from surveys s join answers a on s.survey_id = a.survey_seq_id
where question_uid = 206400374 and insertdate = to_date('31/12/2012');

Answer 1

In answer to your original question:
970992 wrote:
However, when I look at the execution plan, I see that the last predicate (to_number(answer) > 0) has been executed the first. Henceforth, it checks many rows first. Normally, 75 rows belong to 31/12/2012 as you see from the following. Can I specify the execution order?According to the execution plan, it will do a full scan of surveys using the predicate on insertdate to build the (presumably in-memory) hash table (hash based on survey_id) to do the joins (Step 2). Then, it does a full scan of the answers table using the question_uid and answer predicates (Step 3). For each row it finds that matches those predicate, it will prpobe the hash table created in step 2 using the hashed value of survey_seq_id. So, it is doing the insertdate predicate first.
In answer to your last post
970992 wrote:
>
First of all i would get rid of the implizit type conversion:
TO_NUMBER("A"."ANSWER")>0)it is not implicit conversion, I reckon it is explicit type conversion, isnt it?
No, it is an implicit type conversion. Your code says answer > 0, since the answer column is a varchar2 column, Oracle implicitly converts the answer column to a number to compare against the number on the right side of the comparison. Note that if something like 'A' ever becomes a valid answer, then this query will fail with ORA-01722: invalid number.
>
>
Obviously "A"."ANSWER" is not a number colmun, problably varchar, so use something like
A.ANSWER != "0"
or
A.ANSWER > "0"Yes answer column is varchar2 but can you type A.ANSWER > "0" as a predicate? I mean, you can not varchar > varchar, can you?
Of course you can use inequality predicates on a varcahr column. Is the string A greater than the string B?
Based on the explain plan, your statistics might be a little off, not hugely so. The esitmates are at least in the right order of magnitude based on what you have posted so far.
What indexes, if any, are available on the two tables?
John