Why does my dissolved layer draw so slow? And missing features in the result dissolved layer

[TokiLee]

Hello. Is there any way to make my dissolved layer draw faster? I dissolved my 8 million records layer into a 200 features layer instead. But my non-dissolved 8 million polygon features layer draw a lot faster than my 200 dissolved layer. Why? And is there a way to improve the drawing speed? I'm using ArcMap 10.7.1 and QGIS 3.34.11. 

Also, my dissolved layer is missing some areas. Please see attached image, the solid pink is the dissoved version.

Thank you very much!

[George_Thompson]

Think that we need some more information.......

Where is the data being stored?

Did you make sure there are updated indexes on the new layer?

I am guessing that the layer is polygons, correct?

[TokiLee]

Thank you for your recommendation, George!  No, I didn't add spatial index. I'll do and see if it'll help. But would it affect drawing the layer itself though? I'm not even doing the 'Select by Attribute'.  Also, now I have another issue on the dissolved that it's missing some feature/polygon areas. I'm trying to run 'Repair Geometry' now to see if it'd fix it first... Any idea why dissolved layer would have missing areas though? Thanks a lot again! 😃

[VinceAngelo]

Few features with extraordinary numbers of vertices is actually on the other side of the diminishing returns curve, especially if the extents are large, relative to the overall layer extent.

We'd need a lot more information:

• The exact numbers of features (and geometry type)
• The number of vertices and/or parts 
• The storage format (shapefile, file geodatabase, enterprise geodatabase, and if EGDB, which RDBMS)
• The average ratio of the extents to the overall layer extent
• An indication of the extent at which the resulting data is being rendered (relative to overall extent).

- V

[TokiLee]

Thank you Vince for your reply. It is a polygon layer in a feature class within a geodatabase. Before dissolving, there were 7,736,288 polygons, some are overlapping and I'm hoping to use the Dissolve tool to dissolve/melt them together to get the actual area with the 'Calculate geometry' tool after. I assume there are many vertices but I can try to calculate that later but is there a limit I'm looking for each polygon or as a total sum for the layer? Is there a way to minimum vertices? I'm also trying to run the 'Repair Geometry' tool but sometimes it gives me error as well. I think my layer extent is good, but how should I check and what do I look for again? Thanks again! 

[VinceAngelo]

Dissolving millions of features to a few hundred is a nightmare case. You didn't provide most of the requested information necessary to help. Having corrupt geometries before the Dissolve would put a knife into the back of the post-Dissolve topology; that might not be recoverable without significant data loss. 

- V

[DuncanRichards]

Are they multi-part polygons? If so, maybe convert them to single-part polygons. Sometimes ArcMap is slow drawing large complex polygons.

HTH

[TokiLee]

Thanks a lot for your reply, Duncan. I'll try that. I hope to use the 'Dissovle' operation to dissolve/melt the overlaps. Convert them to single-part polygons wouldn't 'unmelt' the features, correct? Because I'd need to calculate the polygon afterward to get the true area. Thanks again! 😃

[RhettZufelt]

The dissolve tool defaults to output of MultiPart polygons.  Make sure you un-select that option also.