Exporting a GeoPackage to Shapefile wrongly changes the field types

[qgib]

Author Name: Jérôme Guélat (Jérôme Guélat)
Original Redmine Issue: 19709
Affected QGIS version: 3.2.2
Redmine category:data_provider/ogr

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Some steps to reproduce the bug:

1. Add the layer contained in the attached GeoPackage. It has 2 fields: the fid (type qlonglong) and another one (type: int)
2. Export the layer as a Shapefile (right-click on the layer -> Export -> Save Features As...)
3. The field types were changed: the fid is now a real (instead of qlonglong), and the other field is now a qlonglong (instead of int)

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• testgeom.gpkg (Jérôme Guélat)

[qgib]

Author Name: Giovanni Manghi (@gioman)

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In straight conversion with ogr2ogr the "att" field of your attachment results in an "int" field as expected.

What about 2.18, worked as expected?

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• crashes_corrupts_data was changed from 1 to 0
• operating_system was changed from Windows 7 to
• status_id was changed from Open to Feedback

[qgib]

Author Name: Jérôme Guélat (Jérôme Guélat)

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Indeed, a conversion with ogr2ogr works as expected... This is a QGIS problem.

The bug also occurs with QGIS 2.18.

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• status_id was changed from Feedback to Open

[qgib]

Author Name: Even Rouault (@rouault)

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Closing as wontfix.
The fundamental problem is that GeoPackage uses binary encoding (int32, int64, float64) whereas Shapefile/DBF uses text encoding. OGR and QGIS have slightly different ways of mapping to DBF field types. For example OGR will map a Int as a Decimal(9) and QGIS as a Decimal(10), and OGR will map a Int64 as a Decimal(18) and QGIS as a Decimal(20).
On reading if a Decimal field has:
<=9 characters, OGR will interpret it as a Int

=10 and <=18 characters, OGR will interpret it as a Int64
=19 characters, OGR will interpret it as a Double

QGIS choice of using Decimal(10) for Int and Decimal(20) for Int64 is actually more correct than ogr/ogr2ogr choice since number likes -1234567890 fit on a Int32 but requires 10 characters.
Perfect round-tripping is impossible given the mismatching between formats

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• resolution was changed from to wontfix

[qgib]

Author Name: Even Rouault (@rouault)

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• status_id was changed from Open to Closed