Filters on Big Object are based on the index fields. Big Object Utility lets you filter records easily using the UI provided in the list view component.
How does the filter work on the index fields?
A filter is always applied from left to right, using the primary index field as first. In the following screenshot, Account__c is the primary index and Customer_Id__c is the secondary index field so when filtering we cannot include the secondary index field without the primary index field.
Q) Why != (Not Equal To) operator is missing?
A) Data retrieval is based on the index field and using != operator doesn't exactly match any index value.
Q) Filters may not have any gaps within the composite key?
A) This error occurs when you are trying to filter without primary index field.