custom picker in Read only grid interface for huge data

Use aggregation queries in your suggest functions to fetch only distinct values (limited to 10 results) and require minimum 2 characters before searching.
Use pagination (50 rows per page) and reset to page 1 when filters change.
The key is that all filtering happens at the database level through the API-backed record, so Appian only processes small filtered result sets, not the full dataset.
Example suggestion function code for your reference.

/* Suggest Function Rule */
a!localVariables(
  /* Query for distinct values using aggregation */
  local!results: a!queryRecordType(
    recordType: recordType!YourRecord,
    fields: {
      a!aggregationFields(
        groupings: a!grouping(
          field: recordType!YourRecord.fields.year,
          alias: "value"
        )
      )
    },
    filters: if(
      len(ri!searchText) > 2,
      a!queryFilter(
        field: recordType!YourRecord.fields.year,
        operator: "starts with",
        value: ri!searchText
      ),
      {}
    ),
    pagingInfo: a!pagingInfo(startIndex: 1, batchSize: 10)
  ),

  /* MUST return a!dataSubset with data and identifiers */
  a!dataSubset(
    data: index(local!results.data, "value", {}),
    identifiers: index(local!results.data, "value", {})
  )
)

Thanks Shubham Aware 

Thanks Shubham Aware