Re: Is it possible to decide the order of where conditions in Flink SQL

Posted by Tony Wei on
URL: http://deprecated-apache-flink-user-mailing-list-archive.369.s1.nabble.com/Is-it-possible-to-decide-the-order-of-where-conditions-in-Flink-SQL-tp29061p29074.html

Hi Hequn,

Thank you very much. It's helpful to me.

For clarification, I think the code should look like the following snippet, since original query was an `AND` operator. Am I right?

CASE
WHEN user_robot THEN false
WHEN !UDF_NEED_TO_QUERY_DB(user) THEN false
ELSE true
END

Best regards,

Tony Wei

Hequn Cheng <[hidden email]> 於 2019年7月28日 週日 下午3:30寫道：

Hi Tony,

There is no order guarantee for filter conditions. The conditions would be pushed down or merged during query optimization. 

However, you can use the case when[1] to achieve what you want. The code looks like:

CASE
WHEN !user.is_robot THEN true
WHEN UDF_NEED_TO_QUERY_DB(user) THEN true
ELSE false
END

For case when, it evaluates the conditions in order.

Note: The UDF_NEED_TO_QUERY_DB must be a nonDeterministic udf, or the case when would also be optimized and changed to an OR by the query optimizer.

You can override the isDeterministic method of ScalarFunction to make it nonDeterministic, i.e., override def isDeterministic: Boolean = false

Best, Hequn

[1] https://ci.apache.org/projects/flink/flink-docs-master/dev/table/functions.html#conditional-functions

On Sat, Jul 27, 2019 at 4:35 PM Tony Wei <[hidden email]> wrote:

Hi,

Thanks for your reply. I have tried both CTE and sql subquery, but it seems that sql plan

optimizer will do filter pushdown. Therefore, where conditions will end up being together in

physical plan.

However, the visualization of physical plans on Flink UI were different for these three SQL

query on their operations' name.

For the original SQL, it showed:

where: (AND(UDF_NEED_TO_QUERY_DB(user), NOT(user.is_robot))), select: (...)

For the CTE and subquery , it showed:

where: (AND(NOT(user.is_robot), UDF_NEED_TO_QUERY_DB(user))), select: (...)

Does this name for each operator of physical plan have any meaning to  represent the

execution order of `where` conditions?

Best,

Tony Wei

sri hari kali charan Tummala <[hidden email]> 於 2019年7月27日 週六 上午3:02寫道：

try cte common table expressions if it supports or sql subquery.

On Fri, Jul 26, 2019 at 1:00 PM Fanbin Bu <[hidden email]> wrote:

how about move query db filter to the outer select.

On Fri, Jul 26, 2019 at 9:31 AM Tony Wei <[hidden email]> wrote:

Hi,

If I have multiple where conditions in my SQL, is it possible to specify its order, so that the query 

can be executed more efficiently?

For example, if I have the following SQL, it used a heavy UDF that needs to access database.

However, if I can specify the order of conditions is executing `!user.is_robot` first then executing

UDF, it will reduce the number of database access. Those records with `true` in `user.is_robot` will

be dropped earlier and don't need to access database. 

select * 

from users

where !user.is_robot and UDF_NEED_TO_QUERY_DB(user)

Thanks,

Tony Wei 

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