Polymorphic vs non-polymorphic query

ECSQL is polymorphic query by default, use the ONLY keyword to make queries non-polymorphic.

Syntax: [ALL|ONLY] <className>

Polymorphic query

SELECT * FORM [BisCore].[GeometricElement3d] Limit 10

-- following is same as above and all GeometricElement3d and its derived classes will be returned.
SELECT * FORM ALL [BisCore].[GeometricElement3d] Limit 10

Non-Polymorphic query

Restrict result to exactly a single type of class.

SELECT * FORM ONLY [BisCore].[GeometricElement3d] Limit 10

ECSql Syntax