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Use expression filter request for SelectByAttribute
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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/
Original file line number Diff line number Diff line change
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('Unsupported field type "%s"' % fields[idx].typeName()))

expression = QgsExpression(expr)

features = vector.features(layer)

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

self.setOutputValue(self.OUTPUT, fileName)

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