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[mssql] Speed up loading of larger views (fixes #27618)

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The extent estimation was taking a long time for views because we can't use
spatial index statistics - the code was doing full scan of the view. Now we
try to scan just 1% of the rows to get a good estimate. If that fails
(e.g. when the table is not very big) we fall back to the full scan.

Test results: a view with 90k rows was taking 2.5s to estimate extents during load - now it's <0.1s
This commit is contained in:
Martin Dobias 2019-10-11 12:38:37 +02:00 committed by Nyall Dawson
parent dfb9914e35
commit ab51de0c3f
1 changed files with 32 additions and 0 deletions
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32
src/providers/mssql/qgsmssqlprovider.cpp
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@ -726,6 +726,10 @@ void QgsMssqlProvider::UpdateStatistics( bool estimate ) const
else
statement = QStringLiteral( "select min(case when ([%1].STIsValid() = 1) THEN [%1].STPointN(1).Long else NULL end), min(case when ([%1].STIsValid() = 1) THEN [%1].STPointN(1).Lat else NULL end), max(case when ([%1].STIsValid() = 1) THEN [%1].STPointN(1).Long else NULL end), max(case when ([%1].STIsValid() = 1) THEN [%1].STPointN(1).Lat else NULL end)" ).arg( mGeometryColName );
}
// we will first try to sample a small portion of the table/view, so the count of rows involved
// will be useful to evaluate if we have enough data to use the sample
statement += ", count(*)";
}
else
{
@ -750,6 +754,34 @@ void QgsMssqlProvider::UpdateStatistics( bool estimate ) const
statement += " where (" + mSqlWhereClause + ')';
}
if ( estimate )
{
// Try to use just 1% sample of the whole table/view to limit the amount of rows accessed.
// This heuristic may fail (e.g. when the table is small or when primary key values do not
// get sampled enough) so in case we do not have at least 10 features, we fall back to full
// traversal of the table/view
const int minSampleCount = 10;
// See https://docs.microsoft.com/en-us/previous-versions/software-testing/cc441928(v=msdn.10)
QString sampleFilter = QString( "(ABS(CAST((BINARY_CHECKSUM([%1])) as int)) % 100) = 42" ).arg( mFidColName );
QString statementSample;
if ( mSqlWhereClause.isEmpty() )
statementSample = statement + " WHERE " + sampleFilter;
else
statementSample = statement + " AND " + sampleFilter;
if ( query.exec( statementSample ) && query.next() &&
!query.value( 0 ).isNull() && query.value( 4 ).toInt() >= minSampleCount )
{
mExtent.setXMinimum( query.value( 0 ).toDouble() );
mExtent.setYMinimum( query.value( 1 ).toDouble() );
mExtent.setXMaximum( query.value( 2 ).toDouble() );
mExtent.setYMaximum( query.value( 3 ).toDouble() );
return;
}
}
if ( !query.exec( statement ) )
{
QgsDebugMsg( query.lastError().text() );