The Index Probe icon indicates that the selection criteria whose predicates matched the leading key columns of the index were used to probe directly into the index. The probe selection will minimize the number of key entries that must be processed and paged into main memory. Additional key selection can be applied against the non-leading key columns of the index to further reduce the number of selected key entries. Only those key entries that matched the specified probe and key selection will be used to select rows from the corresponding table data. If the icon label is Index Probe Distinct or Index Probe Distinct, Preload, then, for each set of duplicate key entries, only one of the entries will be used to select rows from the corresponding table data.
Also referred to as: Index Scan - Key Positioning; Index Probe, Preload; Index Probe Distinct; Index Probe Distinct, Preload
See Tips for index probe for more details.