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- All Known Implementing Classes:
DefaultDiagnosticsListener
public interface DiagnosticsListener
A diagnostics listener.- Author:
- Lukas Eder
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Method Summary
All Methods Instance Methods Abstract Methods Modifier and Type Method Description void
duplicateStatements(DiagnosticsContext ctx)
The executed JDBC statement has duplicates.void
missingWasNullCall(DiagnosticsContext ctx)
The fetched JDBCResultSet
returned a primitive type value for a column, which could have been null, butResultSet.wasNull()
was not called.void
repeatedStatements(DiagnosticsContext ctx)
The executed JDBC statement is repeated consecutively on the same JDBCConnection
.void
tooManyColumnsFetched(DiagnosticsContext ctx)
The fetched JDBCResultSet
returned more columns than necessary.void
tooManyRowsFetched(DiagnosticsContext ctx)
The fetched JDBCResultSet
returned more rows than necessary.void
unnecessaryWasNullCall(DiagnosticsContext ctx)
The fetched JDBCResultSet
returned a value for a column, on whichResultSet.wasNull()
was called unnecessarily (more than once, or for a non-primitive type).
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Method Detail
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tooManyRowsFetched
void tooManyRowsFetched(DiagnosticsContext ctx)
The fetched JDBCResultSet
returned more rows than necessary.An event indicating that a JDBC
ResultSet
was fetched withA
rows, but onlyB
rows (B < A
) were consumed.Typically, this problem can be remedied by applying the appropriate
LIMIT
clause in SQL, orSelectLimitStep.limit(int)
clause in jOOQ.
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tooManyColumnsFetched
void tooManyColumnsFetched(DiagnosticsContext ctx)
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unnecessaryWasNullCall
void unnecessaryWasNullCall(DiagnosticsContext ctx)
The fetched JDBCResultSet
returned a value for a column, on whichResultSet.wasNull()
was called unnecessarily (more than once, or for a non-primitive type).
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missingWasNullCall
void missingWasNullCall(DiagnosticsContext ctx)
The fetched JDBCResultSet
returned a primitive type value for a column, which could have been null, butResultSet.wasNull()
was not called.
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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 sameConnection
) 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.
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repeatedStatements
void repeatedStatements(DiagnosticsContext ctx)
The executed JDBC statement is repeated consecutively on the same JDBCConnection
.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 timeConnection.close()
is called. - From reusing the
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