qgis Virtual layers

• virtual layer
• adding-a-virtual-layer-leads-to-slow-perfomance-and-crashes-in-qgis-3-36-1
  
• qgis-virtual-layers-and-slow-performance
  
• creating-virtual-layers
  
• 慢的无法使用

sql示例：

SELECT gch.rowid as id, gch.geometry as geometry,  gch.L2, gcq.L3, gch.L2 - gcq.L3 as delta_L

FROM  gch

JOIN    gcq 

ON gch.rowid = gcq.rowid;