functions with an array of m counters for each of them The Count-Min Sketch ??? Vasiliki Kalavri | Boston University 2020 21 The Count-Min Sketch m counters h1 h2 hp … p pairwise independent hash functions Kalavri | Boston University 2020 22 for j=1 to p do i = hj(x) ci,j++ Adding an element to the sketch stream elements x All counters are initialized to 0s 0 0 0 6 9 3 3 1 5 0 0 3 8 2 7 9 m counters to decide whether an element is popular • For every element x, we add it to the sketch and then use the updated sketch to estimate its frequency. Computing top-k ??? Vasiliki Kalavri | Boston University

commit cannot be resolved #24384 42 – Fix a GC issue of statistics that might cause duplicated fm-sketch records #24357 – Avoid unnecessary pessimistic rollback when the pessimistic locking receives the restored: • Tables related to statistics: “stats_buckets”, “stats_extended”, “stats_feedback”, “stats_fm_sketch”, “stats_histograms”, “stats_meta”, “stats_top_n” • Tables related to privileges or the system: to- tal num- ber of rows in the ta- ble √ √ Column Count- Min Sketch √ × 598 Information Version 1 Version 2 Index Count- Min Sketch √ × Column Top- N √ √ (Main- te- nance meth- ods and pre- ci-

the feature, TiDB can synchronously load large-sized statistics (such as histograms, TopN, and Count-Min Sketch statistics) into memory when you execute SQL statements, which improves the completeness of restored: • Tables related to statistics: “stats_buckets”, “stats_extended”, “stats_feedback”, “stats_fm_sketch”, “stats_histograms”, “stats_meta”, “stats_top_n” • Tables related to privileges or the system: Version 1 Version 2 The to- tal num- ber of rows in the ta- ble √ √ Column Count- Min Sketch √ × Index Count- Min Sketch √ × Column Top- N √ √ (Main- te- nance meth- ods and pre- ci- sion are im- proved)

gc_delete_range_done | | global_variables | | stats_buckets | | stats_extended | | stats_feedback | | stats_fm_sketch | | stats_histograms | | stats_history | | stats_meta | | stats_meta_history | | stats_table_locked TOPNs. • WITH NUM CMSKETCH DEPTH specifies the depth of the CM Sketch. • WITH NUM CMSKETCH WIDTH specifies the width of the CM Sketch. • WITH NUM SAMPLES specifies the number of samples. 2097 • WITH correspond to two different algo- rithms of collecting samples. See Histograms, Top-N and CMSketch (Count-Min Sketch) for detailed explanations. For SAMPLES/SAMPLERATE, see Improve collection performance. For

gc_delete_range_done | | global_variables | | stats_buckets | | stats_extended | | stats_feedback | | stats_fm_sketch | | stats_histograms | | stats_history | | stats_meta | | stats_meta_history | | stats_table_locked TOPNs. • WITH NUM CMSKETCH DEPTH specifies the depth of the CM Sketch. • WITH NUM CMSKETCH WIDTH specifies the width of the CM Sketch. • WITH NUM SAMPLES specifies the number of samples. 1673 • WITH correspond to two different algo- rithms of collecting samples. See Histograms, Top-N and CMSketch (Count-Min Sketch) for detailed explanations. For SAMPLES/SAMPLERATE, see Improve collection performance. For

gc_delete_range_done | | global_variables | | stats_buckets | | stats_extended | | stats_feedback | | stats_fm_sketch | | stats_histograms | | stats_history | | stats_meta | | stats_meta_history | | stats_table_locked TOPNs. • WITH NUM CMSKETCH DEPTH specifies the depth of the CM Sketch. • WITH NUM CMSKETCH WIDTH specifies the width of the CM Sketch. • WITH NUM SAMPLES specifies the number of samples. 1681 • WITH correspond to two different algo- rithms of collecting samples. See Histograms, Top-N and CMSketch (Count-Min Sketch) for detailed explanations. For SAMPLES/SAMPLERATE, see Improve collection performance. For

gc_delete_range_done | | global_variables | | stats_buckets | | stats_extended | | stats_feedback | | stats_fm_sketch | | stats_histograms | | stats_history | | stats_meta | | stats_meta_history | | stats_table_locked TOPNs. • WITH NUM CMSKETCH DEPTH specifies the depth of the CM Sketch. • WITH NUM CMSKETCH WIDTH specifies the width of the CM Sketch. • WITH NUM SAMPLES specifies the number of samples. 1794 • WITH correspond to two different algo- rithms of collecting samples. See Histograms, Top-N and CMSketch (Count-Min Sketch) for detailed explanations. For SAMPLES/SAMPLERATE, see Improve collection performance. For

gc_delete_range_done | | global_variables | | stats_buckets | | stats_extended | | stats_feedback | | stats_fm_sketch | | stats_histograms | | stats_history | | stats_meta | | stats_meta_history | | stats_table_locked TOPNs. • WITH NUM CMSKETCH DEPTH specifies the depth of the CM Sketch. • WITH NUM CMSKETCH WIDTH specifies the width of the CM Sketch. • WITH NUM SAMPLES specifies the number of samples. 1669 • WITH correspond to two different algo- rithms of collecting samples. See Histograms, Top-N and CMSketch (Count-Min Sketch) for detailed explanations. For SAMPLES/SAMPLERATE, see Improve collection performance. For

| | schema_index_usage | | stats_buckets | | stats_extended | | stats_feedback | | stats_fm_sketch | | stats_histograms | | stats_history | | stats_meta | | stats_meta_history | | stats_table_locked TOPNs. • WITH NUM CMSKETCH DEPTH specifies the depth of the CM Sketch. • WITH NUM CMSKETCH WIDTH specifies the width of the CM Sketch. • WITH NUM SAMPLES specifies the number of samples. 1624 • WITH correspond to two different algo- rithms of collecting samples. See Histograms, Top-N and CMSketch (Count-Min Sketch) for detailed explanations. For SAMPLES/SAMPLERATE, see Improve collection performance. For