Is it possible to disable jpa hints per particular query?

问题

JPA, and in my particular case eclipselink, generates /*+ FIRST_ROWS */ in case of using query.setFirstResult()/query.setMaxResults():

SELECT * FROM (
  SELECT /*+ FIRST_ROWS */ a.*, ROWNUM rnum  FROM (
    SELECT * FROM TABLES INCLUDING JOINS, ORDERING, etc.) a
  WHERE ROWNUM <= 10 )
WHERE rnum > 0;

That forces Oracle to use nested loops instead of hash-joins. In general is has seance, but in my particular case it dramatically decrease performance.

Is it possible to disable hint usage/generation for a particular query?

回答1:

As @ibre5041 told, FIRST_ROWS hint is deprecated, in context of Oracle, FIRST_ROWS(N) should be used instead of it. In my case neither FIRST_ROW nor FIRST_ROW(N) is actually needed, so in order to tell eclipselink not to use outdated stuff, it's possible to specify oracle version within persistence.xml:

<property name="eclipselink.target-database" value="org.eclipse.persistence.platform.database.oracle.Oracle11Platform" />

After adding this, I got strange error: Could not initialize class org.eclipse.persistence.platform.database.oracle.Oracle11Platform However, after I put ojdbcN.jar to domain/lib/ext the error has gone.

As a result, eclipselink generates query without FIRST_ROW hint, and Oracle uses better plan.

来源：https://stackoverflow.com/questions/36840776/is-it-possible-to-disable-jpa-hints-per-particular-query