An attention-grabbing trace by Vladimir Sitnikov has made me take into consideration a brand new benchmark for jOOQ:
The benchmark ought to examine whether or not single row queries ought to have a JDBC Assertion.setFetchSize(1) name made to them by default. The Javadoc of the strategy says:
Offers the JDBC driver a touch as to the variety of rows that ought to be fetched from the database when extra rows are wanted for
ResultSetobjects generated by thisAssertion. If the worth specified is zero, then the trace is ignored. The default worth is zero.
If an ORM (e.g. jOOQ) is aware of that it’s going to fetch only one row, or if it is aware of that there may be only one row, then that trace actually is sensible. Examples in jOOQ embrace:
- When customers name
ResultQuery.fetchSingle(), orfetchOne(), orfetchOptional(), or any related technique, then it’s affordable to count on solely 0 – 1 rows to be returned. Within the case of these strategies returning greater than 1 row, an exception is thrown, so even when there are extra rows, 2 rows will probably be fetched at most. - When customers add a
LIMIT 1clause on a high stage question, there can by no means be greater than 1 row. - When the question is trivial (no joins, or solely to-one joins, no
GROUP BY GROUPING SETS, noUNION, and so on.) and an equality predicate on aUNIQUEconstraint is current, there will also be not more than 1 row.
The database optimiser is aware of all of this stuff as nicely. When you add LIMIT 1 to a question, then the optimiser may be fairly anticipated to take that as a powerful trace in regards to the consequence set measurement. However the JDBC driver doesn’t know this stuff (or a minimum of, it shouldn’t be anticipated to), as a result of it’s unlikely that it parses the SQL and calculates statistics on it, or considers meta information for such optimisations.
So, the person might trace. And since that will be very tedious for customers, even higher, the ORM (e.g. jOOQ) ought to trace. Or so it appears.
Benchmarks
However ought to it? Is it actually definitely worth the hassle? Right here’s Vladimir’s evaluation in regards to the pgjdbc driver, the place he wouldn’t count on an enchancment now, however maybe sooner or later.
Higher than making assumptions, let’s measure, utilizing a JMH benchmark. JMH is generally used for microbenchmarking issues on the JVM, to check assumptions about JIT runtime behaviour. That is clearly not a microbenchmark, however I nonetheless like JMH’s strategy and output, which incorporates customary deviations and errors, in addition to ignores warmup penalties, and so on.
First off, the outcomes:
As a result of benchmark outcomes can’t be printed for some business RDBMS (a minimum of not when evaluating between RDBMS), I’ve normalised the outcomes so a comparability of precise execution velocity between RDBMS isn’t potential. I.e. for every RDBMS, the sooner execution is 1, and the slower one is a few fraction of 1. That approach, the RDBMS is just benchmarked towards itself, which is honest.
The outcomes are beneath. We’re measuring throughput, so decrease is worse.
Db2 --- Benchmark Mode Rating JDBCFetchSizeBenchmark.fetchSize1 thrpt 0.677 JDBCFetchSizeBenchmark.noFetchSize thrpt 1.000 MySQL ----- Benchmark Mode Rating JDBCFetchSizeBenchmark.fetchSize1 thrpt 0.985 JDBCFetchSizeBenchmark.noFetchSize thrpt 1.000 Oracle ------ Benchmark Mode Rating JDBCFetchSizeBenchmark.fetchSize1 thrpt 0.485 JDBCFetchSizeBenchmark.noFetchSize thrpt 1.000 PostgreSQL ---------- Benchmark Mode Rating JDBCFetchSizeBenchmark.fetchSize1 thrpt 1.000 JDBCFetchSizeBenchmark.noFetchSize thrpt 0.998 SQL Server ---------- Benchmark Mode Rating JDBCFetchSizeBenchmark.fetchSize1 thrpt 0.972 JDBCFetchSizeBenchmark.noFetchSize thrpt 1.000
For every RDBMS, I’ve run a trivial question producing a single row with 1 column. Every time, I’ve re-created a JDBC Assertion, and fetched the ResultSet. In fetchSize1, I’ve specified the fetch measurement trace. In noFetchSize, I left the default untouched. As may be summarised:
In these RDBMS, there was no impact
- MySQL
- PostgreSQL
- SQL Server
In these RDBMS, issues obtained considerably worse (not higher!):
That is fairly shocking, because the benchmark consists of working the whole assertion on the server, so I might have anticipated, at finest, a negligible consequence.
For this benchmark, I used to be utilizing these server and JDBC driver variations:
- Db2 11.5.6.0 with jcc-11.5.6.0
- MySQL 8.0.29 with mysql-connector-java-8.0.28
- Oracle 21c with ojdbc11-21.5.0.0
- PostgreSQL 14.1 with postgresql-42.3.3
- SQL Server 2019 with mssql-jdbc-10.2.0
The benchmark logic is right here:
bundle org.jooq.take a look at.benchmarks.native;
import java.sql.*;
import org.openjdk.jmh.annotations.*;
import org.openjdk.jmh.infra.Blackhole;
@Fork(worth = 1)
@Warmup(iterations = 3, time = 3)
@Measurement(iterations = 7, time = 3)
public class JDBCFetchSizeBenchmark {
@State(Scope.Benchmark)
public static class BenchmarkState {
Connection connection;
@Setup(Stage.Trial)
public void setup() throws Exception {
Class.forName("org.postgresql.Driver");
connection = DriverManager.getConnection(
"jdbc:postgresql://localhost:5432/postgres",
"postgres",
"take a look at"
);
}
@TearDown(Stage.Trial)
public void teardown() throws Exception {
connection.shut();
}
}
@FunctionalInterface
interface ThrowingConsumer<T> {
void settle for(T t) throws SQLException;
}
personal void run(
Blackhole blackhole,
BenchmarkState state,
ThrowingConsumer<Assertion> c
) throws SQLException {
attempt (Assertion s = state.connection.createStatement()) {
c.settle for(s);
attempt (ResultSet rs = s.executeQuery(
"choose title from t_book the place id = 1")
) {
whereas (rs.subsequent())
blackhole.eat(rs.getString(1));
}
}
}
@Benchmark
public void fetchSize1(Blackhole blackhole, BenchmarkState state)
throws SQLException {
run(blackhole, state, s -> s.setFetchSize(1));
}
@Benchmark
public void noFetchSize(Blackhole blackhole, BenchmarkState state)
throws SQLException {
run(blackhole, state, s -> {});
}
}
A number of remarks:
- The question is under no circumstances consultant of a manufacturing workload. But when issues did get improved by the
fetchSizeflag, the development ought to have manifested - The benchmark didn’t use ready statements, which might have eliminated some side-effects, or added some side-effects. Be happy to repeat the benchmark utilizing ready statements.
- It isn’t but understood why issues didn’t matter in some drivers, or why they did in others. For the conclusion, the “why” isn’t too essential, as a result of nothing will probably be modified because of this weblog publish. If you already know why (the db2 driver and ojdbc code isn’t open supply, regrettably), I’d be curious.
Conclusion
Optimisations are a difficult beast. Some issues appear to make a number of sense when reasoning about them, however in precise measurements, the seemingly extra optimum factor is definitely worse, or irrelevant.
On this case, at first, it seemed as if we must always trace the JDBC driver about our intentions of fetching only one row. I don’t know why the JDBC driver behaved worse than if I didn’t trace it. Maybe it allotted a buffer that was too small, and needed to enhance it, reasonably than allocating a buffer that was too massive, however massive sufficient. I do know now, because of Douglas Surber’s touch upon the reddit dialogue. The issue is that ojdbc doesn’t know whether or not there will probably be extra rows, so the JDBC rs.subsequent() name has to do one other spherical journey. For extra particulars, see the linked remark above.
I’ve carried out related benchmarks prior to now, attempting to “optimise” preliminary sizes of ArrayList or StringBuilder. I used to be hardly capable of constantly outperform the defaults. Generally, the “enchancment” did appear to enhance issues. Generally, it worsened issues.
With no clear wins (that are understood, don’t blindly belief benchmark outcomes both, even in the event you’re successful!), I misplaced confidence in these enhancements, and didn’t implement them in the long run. This case right here is identical. I’ve not been capable of obtain enhancements, however in 2/5 circumstances, issues obtained considerably worse.
Observe up
On /r/java, there had been a dialogue about this text. It recommended 2 extra checks:
1. Strive utilizing a fetchSize of two
You’d be tempted to suppose that different fetch sizes might nonetheless be applicable, e.g. 2, to stop that potential buffer measurement increment. I simply tried that with Oracle solely, producing:
JDBCFetchSizeBenchmark.fetchSize1 thrpt 0.513 JDBCFetchSizeBenchmark.fetchSize2 thrpt 0.968 JDBCFetchSizeBenchmark.noFetchSize thrpt 1.000
Whereas the penalty of setting the fetchSize to 1 has disappeared, there may be once more no enchancment over the default worth. For a proof, see once more Douglas Surber’s touch upon reddit
2. Strive utilizing PreparedStatements
For my part, PreparedStatement utilization shouldn’t matter for this particular benchmark, which is why I had initially left them out. Somebody on the reddit dialogue was keen to place all their cash on the only PreparedStatement card, so right here’s an up to date consequence, once more with Oracle solely, evaluating static statements with ready ones (up to date benchmark code beneath):
Benchmark Mode Rating JDBCFetchSizeBenchmark.fetchSizePrepared1 thrpt 0.503 JDBCFetchSizeBenchmark.fetchSizeStatic1 thrpt 0.518 JDBCFetchSizeBenchmark.fetchSizePrepared2 thrpt 0.939 JDBCFetchSizeBenchmark.fetchSizeStatic2 thrpt 0.994 JDBCFetchSizeBenchmark.noFetchSizePrepared thrpt 1.000 JDBCFetchSizeBenchmark.noFetchSizeStatic thrpt 0.998
The consequence is identical for each. Not simply that, it may be seen that in my explicit setup (Querying Oracle XE 21c in docker, regionally), there’s completely no distinction between utilizing a static assertion and a ready assertion on this case.
It might once more be attention-grabbing to research why that’s, hypotheses could embrace e.g.
- ojdbc caches additionally static statements within the ready assertion cache
- the impact of caching a ready assertion is negligible in a benchmark that runs solely a single assertion, which is nowhere close to consultant of a manufacturing workload
- the consumer aspect impact of getting ready statements is irrelevant in comparison with the advantages of the cursor cache on the server aspect, or in comparison with the detrimental impact of setting the
fetchSizeto1
The up to date benchmark code:
bundle org.jooq.take a look at.benchmarks.native;
import java.sql.*;
import org.openjdk.jmh.annotations.*;
import org.openjdk.jmh.infra.Blackhole;
@Fork(worth = 1)
@Warmup(iterations = 3, time = 3)
@Measurement(iterations = 7, time = 3)
public class JDBCFetchSizeBenchmark {
@State(Scope.Benchmark)
public static class BenchmarkState {
Connection connection;
@Setup(Stage.Trial)
public void setup() throws Exception {
Class.forName("oracle.jdbc.OracleDriver");
connection = DriverManager.getConnection(
"jdbc:oracle:skinny:@localhost:1521/XEPDB1",
"TEST",
"TEST"
);
}
@TearDown(Stage.Trial)
public void teardown() throws Exception {
connection.shut();
}
}
@FunctionalInterface
interface ThrowingConsumer<T> {
void settle for(T t) throws SQLException;
}
personal void runPrepared(
Blackhole blackhole,
BenchmarkState state,
ThrowingConsumer<Assertion> c
) throws SQLException {
attempt (PreparedStatement s = state.connection.prepareStatement(
"choose title from t_book the place id = 1")
) {
c.settle for(s);
attempt (ResultSet rs = s.executeQuery()) {
whereas (rs.subsequent())
blackhole.eat(rs.getString(1));
}
}
}
personal void runStatic(
Blackhole blackhole,
BenchmarkState state,
ThrowingConsumer<Assertion> c
) throws SQLException {
attempt (Assertion s = state.connection.createStatement()) {
c.settle for(s);
attempt (ResultSet rs = s.executeQuery(
"choose title from t_book the place id = 1")
) {
whereas (rs.subsequent())
blackhole.eat(rs.getString(1));
}
}
}
@Benchmark
public void fetchSizeStatic1(Blackhole blackhole, BenchmarkState state)
throws SQLException {
runStatic(blackhole, state, s -> s.setFetchSize(1));
}
@Benchmark
public void fetchSizeStatic2(Blackhole blackhole, BenchmarkState state)
throws SQLException {
runStatic(blackhole, state, s -> s.setFetchSize(2));
}
@Benchmark
public void noFetchSizeStatic(Blackhole blackhole, BenchmarkState state)
throws SQLException {
runStatic(blackhole, state, s -> {});
}
@Benchmark
public void fetchSizePrepared1(Blackhole blackhole, BenchmarkState state)
throws SQLException {
runPrepared(blackhole, state, s -> s.setFetchSize(1));
}
@Benchmark
public void fetchSizePrepared2(Blackhole blackhole, BenchmarkState state)
throws SQLException {
runPrepared(blackhole, state, s -> s.setFetchSize(2));
}
@Benchmark
public void noFetchSizePrepared(Blackhole blackhole, BenchmarkState state)
throws SQLException {
runPrepared(blackhole, state, s -> {});
}
}
