Allow importing data into existing tables with processing

[qgib]

Author Name: Robert Collins (@rbtcollins)
Original Redmine Issue: 19210
Affected QGIS version: 3.0.3
Redmine category:data_provider/postgis

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When importing a layer that needed some schema tweaks (specifically to change the geom to a plain 'geometry' to avoid Polygon vs MultiPolygon errors... I wanted to turn off layer overwriting.

The ImportIntoPostGIS processing tool passes the overwrite option to
qgsvectorlayerexporter which passes it to
QgsPostgresProvider::createEmptyLayer

but as you can see here

QGIS/src/providers/postgres/qgspostgresprovider.cpp

Line 3878 in 0a325f1

if ( exists && overwrite )

and then

QGIS/src/providers/postgres/qgspostgresprovider.cpp

Line 3893 in 0a325f1

sql = QStringLiteral( "CREATE TABLE %1(" ) .arg( schemaTableName );

the PostGIS driver only uses the overwrite flag to skip dropping the existing table (good), but not to avoid creating a new table (bad) - and this naturally fails.

This seems to be the case even very far back in history, so perhaps this should be considered a feature request rather than a bug?

[m-kuhn]

See qgis/QGIS-Enhancement-Proposals#89

[alexbruy]

Already possible with master.