org.jooq

Interface DiagnosticsListener

All Known Implementing Classes:

DefaultDiagnosticsListener

public interface DiagnosticsListener

A diagnostics listener.

Author:

Lukas Eder

Method Summary

Modifier and Type	Method and Description
void	duplicateStatements(DiagnosticsContext ctx) The executed JDBC statement has duplicates.
void	missingWasNullCall(DiagnosticsContext ctx) The fetched JDBC ResultSet returned a primitive type value for a column, which could have been null, but ResultSet.wasNull() was not called.
void	repeatedStatements(DiagnosticsContext ctx) The executed JDBC statement is repeated consecutively on the same JDBC Connection.
void	tooManyColumnsFetched(DiagnosticsContext ctx) The fetched JDBC ResultSet returned more columns than necessary.
void	tooManyRowsFetched(DiagnosticsContext ctx) The fetched JDBC ResultSet returned more rows than necessary.
void	unnecessaryWasNullCall(DiagnosticsContext ctx) The fetched JDBC ResultSet returned a value for a column, on which ResultSet.wasNull() was called unnecessarily (more than once, or for a non-primitive type).

Method Detail

tooManyRowsFetched

void tooManyRowsFetched(DiagnosticsContext ctx)

The fetched JDBC ResultSet returned more rows than necessary.

An event indicating that a JDBC ResultSet was fetched with A rows, but only B rows (B < A) were consumed.

Typically, this problem can be remedied by applying the appropriate LIMIT clause in SQL, or SelectLimitStep.limit(int) clause in jOOQ.

tooManyColumnsFetched

void tooManyColumnsFetched(DiagnosticsContext ctx)

The fetched JDBC ResultSet returned more columns than necessary.

An event indicating that a JDBC ResultSet was fetched with A columns, but only B (B < A) were consumed.

Typically, this problem can be remedied by not running a SELECT * query when this isn't strictly required.

unnecessaryWasNullCall

void unnecessaryWasNullCall(DiagnosticsContext ctx)

The fetched JDBC ResultSet returned a value for a column, on which ResultSet.wasNull() was called unnecessarily (more than once, or for a non-primitive type).

missingWasNullCall

void missingWasNullCall(DiagnosticsContext ctx)

The fetched JDBC ResultSet returned a primitive type value for a column, which could have been null, but ResultSet.wasNull() was not called.

duplicateStatements

void duplicateStatements(DiagnosticsContext ctx)

The executed JDBC statement has duplicates.

Many databases maintain an execution plan cache, which remembers execution plans for a given SQL string. These caches often use the verbatim SQL string (or a hash thereof) as a key, meaning that "similar" but not identical statements will produce different keys. This may be desired in rare cases when querying skewed data, as a hack to force the optimiser to calculate a new plan for a given "similar" but not identical query, but mostly, this is not desirable as calculating execution plans can turn out to be expensive.

Examples of such duplicate statements include:

Whitespace differences

 SELECT * FROM  actor;
 SELECT  * FROM actor;
 

Inline bind values

 SELECT * FROM actor WHERE id = 1;
 SELECT * FROM actor WHERE id = 2;
 

Aliasing and qualification

 SELECT a1.* FROM actor a1 WHERE id = ?;
 SELECT * FROM actor a2 WHERE a2.id = ?;
 

Examples of identical statements (which are not considered duplicate, but repeatedStatements(DiagnosticsContext), if on the same Connection) are:

 SELECT * FROM actor WHERE id = ?;
 SELECT * FROM actor WHERE id = ?;
 

This is a system-wide diagnostic that is not specific to individual Connection instances.

repeatedStatements

void repeatedStatements(DiagnosticsContext ctx)

The executed JDBC statement is repeated consecutively on the same JDBC Connection.

This problem goes by many names, the most famous one being the N + 1 problem, when a single (1) query for a parent entity requires many (N) subsequent queries for child entities. This could have been prevented by rewriting the parent query to use a JOIN. If such a rewrite is not possible (or not easy), the subsequent N queries could at least profit (depending on the exact query):

• From reusing the PreparedStatement
• From being batched
• From being re-written as a bulk fetch or write query

This problem can be aggravated if combined with the duplicateStatements(DiagnosticsContext) problem, in case of which the repeated statements might not be diagnosed as easily.

Repeated statements may or may not be "identical". In the following example, there are two repeated and identical statements:

 SELECT * FROM actor WHERE id = ?;
 SELECT * FROM actor WHERE id = ?;
 

In this example, we have three repeated statements, only some of which are also identical:

 SELECT * FROM actor WHERE id = ?;
 SELECT * FROM actor WHERE id = ?;
 SELECT * FROM actor WHERE id =  ?;
 

This is a Connection-specific diagnostic that is reset every time Connection.close() is called.