qgspostgresprovider: Fix primary key retrieval with lowercase option

The `lowercaseFieldNames` allows to transform the column names to
lowercase. If this option is enabled, then the requested primary
key (`primaryKey`) is in lowercase but the `fields` are
not. Therefore, the requested primary key will never be found.

This issues is fixed by converting the field names to lowercase when
looking for the primary key if the option is enabled.

Closes: qgis#55856

src/providers/postgres/qgspostgresprovider.cpp

@@ -4609,13 +4609,15 @@ Qgis::VectorExportResult QgsPostgresProvider::createEmptyLayer( const QString &u
   {
     pkList = parseUriKey( primaryKey );
     const auto constPkList = pkList;
+    const bool lowercaseFieldNames = options && options->value( QStringLiteral( "lowercaseFieldNames" ), false ).toBool();
     for ( const QString &col : constPkList )
     {
       // search for the passed field
       QString type;
       for ( int fldIdx = 0; fldIdx < fields.count(); ++fldIdx )
       {
-        if ( fields[fldIdx].name() == col )
+        const QString fieldName = lowercaseFieldNames ? fields[fldIdx].name().toLower() : fields[fldIdx].name();
+        if ( fieldName == col )
         {
           // found, get the field type
           QgsField fld = fields[fldIdx];

tests/src/python/test_provider_postgres.py

@@ -3266,6 +3266,50 @@ def testGeographyAddFeature(self):
         self.assertTrue(dp.addFeature(f))
         self.assertEqual(vl.featureCount(), 1)
 
+    # See: https://github.com/qgis/QGIS/issues/55856
+    def testPktLowerCase(self):
+        # check that primary key creation correctly works
+        # when exporting a vector layer to postgresql with
+        # lowercaseFieldNames option set to True
+
+        # create an empty vector layer
+        pk_key = "DEP"
+        input_uri = f"NoGeometry?crs=&field={pk_key}:string(255,0)&field=REG:string(255,0)&field=Number:integer(10,0)"
+        layer = QgsVectorLayer(input_uri, "lowercase", "memory")
+        self.assertTrue(layer.isValid())
+
+        # export the vector layer to postgresql with lowercase field names
+        self.execSQLCommand('DROP TABLE IF EXISTS qgis_test.pk_lowercase')
+        output_uri = f'{self.dbconn} table="qgis_test"."pk_lowercase" key=\'{pk_key.lower()}\''
+        err = QgsVectorLayerExporter.exportLayer(layer, output_uri, "postgres", layer.crs(), False, {'lowercaseFieldNames': True})
+        self.assertEqual(err[0], QgsVectorLayerExporter.ExportError.NoError,
+                         f'unexpected import error {err}')
+
+        # retrieve the columns and type and check them
+        cur = self.con.cursor()
+        sql_cols = (
+            "SELECT column_name, data_type FROM information_schema.columns "
+            "WHERE table_name = 'pk_lowercase' AND table_schema = 'qgis_test';"
+        )
+        cur.execute(sql_cols)
+        expected_cols = [
+            ('dep', 'character varying'),
+            ('reg', 'character varying'),
+            ('number', 'integer')]
+        self.assertEqual(cur.fetchall(), expected_cols)
+
+        # Retrieve the primary key and check its name and type
+        sql_pk = (
+            "SELECT a.attname, format_type(a.atttypid, a.atttypmod) AS data_type "
+            "FROM   pg_index i "
+            "JOIN   pg_attribute a ON a.attrelid = i.indrelid "
+            "AND a.attnum = ANY(i.indkey) "
+            "WHERE  i.indrelid = 'qgis_test.pk_lowercase'::regclass "
+            "AND    i.indisprimary;"
+        )
+        cur.execute(sql_pk)
+        self.assertEqual(cur.fetchall(), [('dep', 'character varying')])
+
 
 class TestPyQgsPostgresProviderCompoundKey(QgisTestCase, ProviderTestCase):