psql: Improve performance of record()v5.10.1

In order to save having to always run a query to get the tablename for
a known oid then we cache the result on the driver side. The oid stays
the same while the table exists, so only on dropping it would it change.
Recreating the table causes it to get a new oid, so there is no risk of
the old one being associated with the wrong table when this happens, if
the driver is still open at that point.

The benchmark added shows the improvement from the previous code, before
the results for PostgreSQL was:

RESULT : tst_QSqlRecord::benchmarkRecord():"0_QPSQL@localhost":
     259 msecs per iteration (total: 259, iterations: 1)

whereas now it is:

RESULT : tst_QSqlRecord::benchmarkRecord():"0_QPSQL@localhost":
     0.000014 msecs per iteration (total: 59, iterations: 4194304)

Task-number: QTBUG-65226
Change-Id: Ic290cff719102743da84e2044cd23e540f20c96c
Reviewed-by: Christian Ehrlicher <ch.ehrlicher@gmx.de>
Reviewed-by: Robert Szefner <robertsz27@interia.pl>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 10 insertions, 4 deletions

diff --git a/src/plugins/sqldrivers/psql/qsql_psql.cpp b/src/plugins/sqldrivers/psql/qsql_psql.cpp
index 35b0f9a3e3..c381572713 100644
--- a/src/plugins/sqldrivers/psql/qsql_psql.cpp
+++ b/src/plugins/sqldrivers/psql/qsql_psql.cpp
@@ -183,6 +183,7 @@ public:
     void setDatestyle();
     void setByteaOutput();
     void detectBackslashEscape();
+    mutable QHash<int, QString> oidToTable;
 };
 
 void QPSQLDriverPrivate::appendTables(QStringList &tl, QSqlQuery &t, QChar type)
@@ -553,11 +554,16 @@ QSqlRecord QPSQLResult::record() const
             f.setName(QString::fromUtf8(PQfname(d->result, i)));
         else
             f.setName(QString::fromLocal8Bit(PQfname(d->result, i)));
-        QSqlQuery qry(driver()->createResult());
-        if (qry.exec(QStringLiteral("SELECT relname FROM pg_class WHERE pg_class.oid = %1")
-                     .arg(PQftable(d->result, i))) && qry.next()) {
-            f.setTableName(qry.value(0).toString());
+        const int tableOid = PQftable(d->result, i);
+        auto &tableName = d->drv_d_func()->oidToTable[tableOid];
+        if (tableName.isEmpty()) {
+            QSqlQuery qry(driver()->createResult());
+            if (qry.exec(QStringLiteral("SELECT relname FROM pg_class WHERE pg_class.oid = %1")
+                         .arg(tableOid)) && qry.next()) {
+                tableName = qry.value(0).toString();
+            }
         }
+        f.setTableName(tableName);
         int ptype = PQftype(d->result, i);
         f.setType(qDecodePSQLType(ptype));
         int len = PQfsize(d->result, i);