Remove temporary string allocations when reading prepared statement.

Instead, we use the binary MySQL encoding and copy the data directly
into the QVariant of the desired type. This gets rid of the temporary
string allocations and greatly improves the performance of the added
benchmark. On my machine, the results are:

Before:
     0.562 msecs per iteration (total: 563, iterations: 1000)
     1,922,479.330 instructions per iteration (total: 1,922,479,330, iterations: 1000)

After:
     0.381 msecs per iteration (total: 381, iterations: 1000)
     774,132.957 instructions per iteration (total: 774,132,958, iterations: 1000)

Note that the same could be applied to floating point data types in
the future. Additionally, special support for MYSQL_TIME structure
coult be added to get rid of the string conversions there.

To ensure everything keeps working, a new auto test is added as well
that verifies the select statements and insertions of integral data
into a MySql table works as intended.

[ChangeLog][QtSql] Improve performance when reading integer values
from MySQL databases via prepared statements.

Change-Id: I21dd9277661971ded934546f09535014b63f8eb8
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

1 files changed, 40 insertions, 0 deletions

diff --git a/tests/benchmarks/sql/kernel/qsqlquery/main.cpp b/tests/benchmarks/sql/kernel/qsqlquery/main.cpp
index 63aa6f7250..b5937b76b0 100644
--- a/tests/benchmarks/sql/kernel/qsqlquery/main.cpp
+++ b/tests/benchmarks/sql/kernel/qsqlquery/main.cpp
@@ -55,6 +55,8 @@ public slots:
 private slots:
     void benchmark_data() { generic_data(); }
     void benchmark();
+    void benchmarkSelectPrepared_data() { generic_data(); }
+    void benchmarkSelectPrepared();
 
 private:
     // returns all database connections
@@ -264,4 +266,42 @@ void tst_QSqlQuery::benchmark()
     tst_Databases::safeDropTable( db, tableName );
 }
 
+void tst_QSqlQuery::benchmarkSelectPrepared()
+{
+    QFETCH( QString, dbName );
+    QSqlDatabase db = QSqlDatabase::database(dbName);
+    CHECK_DATABASE(db);
+    if (tst_Databases::getMySqlVersion(db).section(QChar('.'), 0, 0).toInt() < 5)
+        QSKIP("Test requires MySQL >= 5.0");
+
+    QSqlQuery q(db);
+    const QString tableName(qTableName("benchmark", __FILE__, db));
+
+    tst_Databases::safeDropTable(db, tableName);
+
+    QVERIFY_SQL(q, exec("CREATE TABLE " + tableName + "(id INT NOT NULL)"));
+
+    const int NUM_ROWS = 1000;
+    int expectedSum = 0;
+    QString fillQuery = "INSERT INTO " + tableName + " VALUES (0)";
+    for (int i = 1; i < NUM_ROWS; ++i) {
+        fillQuery += ", (" + QString::number(i) + ")";
+        expectedSum += i;
+    }
+    QVERIFY_SQL(q, exec(fillQuery));
+
+    QVERIFY_SQL(q, prepare("SELECT id FROM "+tableName));
+    QBENCHMARK {
+        QVERIFY_SQL(q, exec());
+        int sum = 0;
+
+        while (q.next())
+            sum += q.value(0).toInt();
+
+        QCOMPARE(sum, expectedSum);
+    }
+
+    tst_Databases::safeDropTable(db, tableName);
+}
+
 #include "main.moc"