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Use expression filter request for SelectByAttribute
Makes SelectByAttribute orders of magnitude faster for providers which
support compiled expressions (eg Postgres), also allows algorithm
to take advantage of database indexes created on attribute.

For a 4 million point PostGIS layer (with an index on attribute)
BEFORE: cancelled after 20 mins
AFTER: ~2 seconds
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nyalldawson committed Nov 13, 2015
1 parent fd9f0b5 commit 7c8177e
Showing 1 changed file with 7 additions and 13 deletions.
20 changes: 7 additions & 13 deletions python/plugins/processing/algs/qgis/SelectByAttribute.py
Expand Up @@ -26,7 +26,7 @@
__revision__ = '$Format:%H$'

from PyQt4.QtCore import QVariant
from qgis.core import QgsExpression
from qgis.core import QgsExpression, QgsFeatureRequest
from processing.core.GeoAlgorithm import GeoAlgorithm
from processing.core.GeoAlgorithmExecutionException import GeoAlgorithmExecutionException
from processing.core.parameters import ParameterVector
Expand Down Expand Up @@ -115,18 +115,12 @@ def processAlgorithm(self, progress):
raise GeoAlgorithmExecutionException(
self.tr('Unsupported field type "%s"' % fields[idx].typeName()))

expression = QgsExpression(expr)
expression.prepare(fields)

features = vector.features(layer)

selected = []
count = len(features)
total = 100.0 / float(count)
for count, f in enumerate(features):
if expression.evaluate(f, fields):
selected.append(f.id())
progress.setPercentage(int(count * total))
qExp = QgsExpression(expr)
if not qExp.hasParserError():
qReq = QgsFeatureRequest(qExp)
else:
raise GeoAlgorithmExecutionException(qExp.parserErrorString())
selected = [f.id() for f in layer.getFeatures(qReq)]

layer.setSelectedFeatures(selected)
self.setOutputValue(self.OUTPUT, fileName)

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