How Spark compares data of type boolean and string-CodePudding

SQL:

SELECT 'true' = true;

I know the result is true, but I want to know how Spark compares the two：

cast true as 'true' and then just compare two string type.
cast 'true' as true and then compare two boolean type.

Which way does Spark use?

The same happens when comparing date and string or timestamp and string:

SELECT '2022-01-02' > date('2022-01-03');
SELECT '2022-08-12 01:00:00.123456' < timestamp('2019-08-12 01:00:00.123456');

I'm a novice at Spark. Thank you in advance！

CodePudding user response：

The easiest way to learn is to ask Spark to explain the plan.

scala> spark.sql("SELECT 'true' = true").explain
== Physical Plan ==
*(1) Project [true AS (CAST(true AS BOOLEAN) = true)#53]
 - *(1) Scan OneRowRelation[]

scala> spark.sql("SELECT '2022-01-02' > date('2022-01-03')").explain
== Physical Plan ==
*(1) Project [false AS (CAST(2022-01-02 AS DATE) > CAST(2022-01-03 AS DATE))#55]
 - *(1) Scan OneRowRelation[]

scala> spark.sql("SELECT '2022-08-12 01:00:00.123456' < timestamp('2019-08-12 01:00:00.123456')").explain
== Physical Plan ==
*(1) Project [false AS (CAST(2022-08-12 01:00:00.123456 AS TIMESTAMP) < CAST(2019-08-12 01:00:00.123456 AS TIMESTAMP))#57]
 - *(1) Scan OneRowRelation[]

We can clearly see that in all cases Spark attempts to cast string to the other type (boolean, date, timestamp, int, ...).