Dissolve

A dissolve step merges every input geometry into a single feature with PostGIS ST_Union. There is no key field and no attribute aggregation on this path: the output has empty properties.

Use when you want one combined shape — a total extent, a boundary-cleanup union of same-area tiles you already filtered, or a single mask polygon for a later clip / erase.

If you need one polygon per attribute value (land-use categories, ownership, zoning) plus counts or sums, use Group by (always ST_Union per group) instead. That is the shipped keyed path.

Inputs

• Source layer. Any geometry type. Null geometries are skipped. The step has no parameters in the derived-layer builder.

Output

Exactly one feature (when the source has at least one non-null geometry):

• Geometry: the union of every input geometry (often a multipolygon when parts do not touch).
• Properties: empty ({}). Source attributes are not carried forward — there is no canonical way to pick which name wins among dozens of input rows.

Downstream steps that need a named field will fail validation if they sit after dissolve. Filter or calculate before dissolving, or use group-by when you need keys and aggregates.

How to use it on its own

From Analyze (guided)

1. Open a data layer and switch to the Analyze tab.
2. Click Dissolve this layer.
3. Optionally edit the title.
4. Click Create derived layer.

The portal creates a private derived layer and opens it. Expect a short server job, not an instant browser result. Attributes are dropped on this path.

From the new-item wizard

1. New-item wizard → Derived layer.
2. Pick a Source.
3. Add a Dissolve step (no options to fill in).
4. Save.

The builder shows a short note that attributes are dropped.

Examples

• Single overall extent. Source: any polygon layer. Output: one multipolygon that is the union of every input. Useful as a quick "what's the total coverage" reference or as a clip mask.
• Pre-filtered union. Filter parcels to one owner first, then dissolve — one shape for that owner without inventing a key field on dissolve itself.

Dissolve vs. group-by

Both can merge geometries. The differences:

• Dissolve (this step) is the simple path: one union of the whole input, no attributes. Prefer it when that is all you need.
• Group by is the path when you also want group keys and aggregates (count, sum, average, …) alongside ST_Union geometry per key.

Notes

• Non-adjacent parts become multipolygons. Scattered parcels that you dissolve together produce one multipolygon feature.
• Internal holes can survive when they are rings in the unioned geometry.
• Slow on huge inputs. Dissolving millions of polygons can take minutes; filter first or run off-hours.
• No key-field dissolve here. That wording in older docs was wrong for this generator; use group-by.