# Histogram - Cont'd + The sample table `T2` has 19,500 of identical rows + By default, the histogram is comprised of 256 buckets. + The `Count` column reflects a cumulative total + `Ndv` (number of distinct values) is provided for the current bucket when `tidb_analyze_version = 2` is enabled. ``` tidb> SHOW ~!@STATS_BUCKETS@!~ WHERE db_name = 'test' AND table_name = 'T2'; +---------+------------+----------------+-------------+----------+-----------+-------+---------+-------------+-------------+------+ | Db_name | Table_name | Partition_name | Column_name | Is_index | Bucket_id | Count | Repeats | Lower_Bound | Upper_Bound | Ndv | +---------+------------+----------------+-------------+----------+-----------+-------+---------+-------------+-------------+------+ | test | T2 | | id | 0 | 0 | 77 | 1 | 501 | 577 | 0 | | test | T2 | | id | 0 | 1 | 154 | 1 | 578 | 654 | 0 | | test | T2 | | id | 0 | 2 | 231 | 1 | 655 | 731 | 0 | | test | T2 | | id | 0 | 3 | 308 | 1 | 732 | 808 | 0 | | test | T2 | | id | 0 | 4 | 385 | 1 | 809 | 885 | 0 | | test | T2 | | id | 0 | 5 | 462 | 1 | 886 | 962 | 0 | | test | T2 | | id | 0 | 6 | 539 | 1 | 963 | 1039 | 0 | ... | test | T2 | | id | 0 | 248 | 19173 | 1 | 19597 | 19673 | 0 | | test | T2 | | id | 0 | 249 | 19250 | 1 | 19674 | 19750 | 0 | | test | T2 | | id | 0 | 250 | 19327 | 1 | 19751 | 19827 | 0 | | test | T2 | | id | 0 | 251 | 19404 | 1 | 19828 | 19904 | 0 | | test | T2 | | id | 0 | 252 | 19481 | 1 | 19905 | 19981 | 0 | | test | T2 | | id | 0 | 253 | 19500 | 1 | 19982 | 20000 | 0 | +---------+------------+----------------+-------------+----------+-----------+-------+---------+-------------+-------------+------+ 254 rows in set (0.00 sec) ``` ------------------------------------------------------------------------------ ``` tidb> SHOW CREATE TABLE T2\G *************************** 1. row *************************** Table: T2 Create Table: CREATE TABLE `T2` ( `id` int(11) NOT NULL AUTO_INCREMENT, `pname` varchar(32) DEFAULT NULL, `price` varchar(32) DEFAULT NULL, `info` varchar(32) DEFAULT NULL, PRIMARY KEY (`id`) /*T![clustered_index] CLUSTERED */, KEY `idx_pname` (`pname`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin AUTO_INCREMENT=30001 1 row in set (0.00 sec) tidb> SELECT * FROM T2 LIMIT 10; +----+-------+-------+------+ | id | pname | price | info | +----+-------+-------+------+ | 1 | 000 | 9999 | none | | 2 | 000 | 9999 | none | | 3 | 000 | 9999 | none | | 4 | 000 | 9999 | none | | 5 | 000 | 9999 | none | | 6 | 000 | 9999 | none | | 7 | 000 | 9999 | none | | 8 | 000 | 9999 | none | | 9 | 000 | 9999 | none | | 10 | 000 | 9999 | none | +----+-------+-------+------+ 10 rows in set (0.00 sec) tidb> SHOW STATS_BUCKETS WHERE db_name = 'test' AND table_name = 'T2'; +---------+------------+----------------+-------------+----------+-----------+-------+---------+-------------+-------------+------+ | Db_name | Table_name | Partition_name | Column_name | Is_index | Bucket_id | Count | Repeats | Lower_Bound | Upper_Bound | Ndv | +---------+------------+----------------+-------------+----------+-----------+-------+---------+-------------+-------------+------+ | test | T2 | | id | 0 | 0 | 77 | 1 | 501 | 577 | 0 | | test | T2 | | id | 0 | 1 | 154 | 1 | 578 | 654 | 0 | | test | T2 | | id | 0 | 2 | 231 | 1 | 655 | 731 | 0 | | test | T2 | | id | 0 | 3 | 308 | 1 | 732 | 808 | 0 | | test | T2 | | id | 0 | 4 | 385 | 1 | 809 | 885 | 0 | | test | T2 | | id | 0 | 5 | 462 | 1 | 886 | 962 | 0 | | test | T2 | | id | 0 | 6 | 539 | 1 | 963 | 1039 | 0 | ... | test | T2 | | id | 0 | 248 | 19173 | 1 | 19597 | 19673 | 0 | | test | T2 | | id | 0 | 249 | 19250 | 1 | 19674 | 19750 | 0 | | test | T2 | | id | 0 | 250 | 19327 | 1 | 19751 | 19827 | 0 | | test | T2 | | id | 0 | 251 | 19404 | 1 | 19828 | 19904 | 0 | | test | T2 | | id | 0 | 252 | 19481 | 1 | 19905 | 19981 | 0 | | test | T2 | | id | 0 | 253 | 19500 | 1 | 19982 | 20000 | 0 | +---------+------------+----------------+-------------+----------+-----------+-------+---------+-------------+-------------+------+ 254 rows in set (0.00 sec) ``` The `id` column in table T2 serves as the primary key, with a secondary index `idx_pname` on the `pname` column. Currently, T2 contains 19,500 rows of data, where each row features a unique value in the `id` column, while values in other columns remain identical across all rows. To examine the details of each bucket in the histogram, we execute the command `SHOW STATS_BUCKETS WHERE db_name = 'test' AND table_name = 'T2'`. By default, the histogram is comprised of 256 buckets. From the output, the first row’s `Column_name` indicates the column `id`. The `Bucket_id` denotes the bucket number, while the `Count` specifies the total number of values within the bucket (representing the histogram's depth). The `Lower_Bound` marks the smallest value, and the `Upper_Bound` the largest value in the bucket, delineating the histogram's range. The `repeats` field quantifies the occurrences of the maximum value within the bucket, and the `ndv` (number of distinct values) is provided for the current bucket when `tidb_analyze_version = 2` is enabled. It's important to note that although one might expect the `Count` value to remain constant across an equal-depth histogram, the values are visibly increasing here. This is because the `Count` column reflects a cumulative total, where each entry corresponds to the aggregate of the current and all preceding rows. Histograms thus play a crucial role in index selection, particularly for determining indexes during index range scans. For an in-depth explanation of the `SHOW STATS_BUCKETS` command, please visit: [Buckets of Histogram Documentation](https://docs.pingcap.com/tidb/stable/statistics#buckets-of-histogram).