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information-schema: note the extra cost of querying TABLE/PARTITION size columns #23685

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information-schema: note the extra cost of querying TABLE/PARTITION size columns #23685

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4 changes: 4 additions & 0 deletions information-schema/information-schema-partitions.md
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Expand Up @@ -7,6 +7,10 @@ summary: Learn the `PARTITIONS` INFORMATION_SCHEMA table.

The `PARTITIONS` table provides information about [partitioned tables](/partitioned-table.md).

> **Note:**
>
> Computing `AVG_ROW_LENGTH`, `DATA_LENGTH`, or `INDEX_LENGTH` requires TiDB to read per-column size statistics from the `mysql.stats_histograms` system table, which holds one row for each column of every table. On clusters with a large number of tables, selecting any of these columns can consume noticeably more time and resources than selecting only `TABLE_ROWS`, which reads only the smaller `mysql.stats_meta` system table. If you need only the estimated row count, select `TABLE_ROWS` without the size columns to avoid this overhead.

@qiancai qiancai Sep 3, 2026

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> Computing `AVG_ROW_LENGTH`, `DATA_LENGTH`, or `INDEX_LENGTH` requires TiDB to read per-column size statistics from the `mysql.stats_histograms` system table, which holds one row for each column of every table. On clusters with a large number of tables, selecting any of these columns can consume noticeably more time and resources than selecting only `TABLE_ROWS`, which reads only the smaller `mysql.stats_meta` system table. If you need only the estimated row count, select `TABLE_ROWS` without the size columns to avoid this overhead.
> Querying `AVG_ROW_LENGTH`, `DATA_LENGTH`, or `INDEX_LENGTH` requires TiDB to read column-size statistics from `mysql.stats_histograms`. On clusters with a large number of tables, selecting any of these columns can consume more time and resources than selecting only `TABLE_ROWS`, which only requires row-count statistics from `mysql.stats_meta`. If you need only the estimated row count, select `TABLE_ROWS` without the size columns to avoid this overhead.

@qiancai qiancai Sep 3, 2026

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reason: to focus on the user-visible performance impact. The exact layout of mysql.stats_histograms is an implementation detail, and saying it holds one row for each column of every table is too absolute because the table also stores index statistics.


```sql
USE INFORMATION_SCHEMA;
DESC partitions;
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4 changes: 4 additions & 0 deletions information-schema/information-schema-tables.md
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Expand Up @@ -123,6 +123,10 @@ The description of columns in the `TABLES` table is as follows:
* `CREATE_OPTIONS`: Creates options.
* `TABLE_COMMENT`: The comments and notes of the table.

> **Note:**
>
> Computing `AVG_ROW_LENGTH`, `DATA_LENGTH`, or `INDEX_LENGTH` requires TiDB to read per-column size statistics from the `mysql.stats_histograms` system table, which holds one row for each column of every table. On clusters with a large number of tables, selecting any of these columns can consume noticeably more time and resources than selecting only `TABLE_ROWS`, which reads only the smaller `mysql.stats_meta` system table. If you need only the estimated row count, select `TABLE_ROWS` without the size columns to avoid this overhead.

@qiancai qiancai Sep 3, 2026

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> Computing `AVG_ROW_LENGTH`, `DATA_LENGTH`, or `INDEX_LENGTH` requires TiDB to read per-column size statistics from the `mysql.stats_histograms` system table, which holds one row for each column of every table. On clusters with a large number of tables, selecting any of these columns can consume noticeably more time and resources than selecting only `TABLE_ROWS`, which reads only the smaller `mysql.stats_meta` system table. If you need only the estimated row count, select `TABLE_ROWS` without the size columns to avoid this overhead.
> Querying `AVG_ROW_LENGTH`, `DATA_LENGTH`, or `INDEX_LENGTH` requires TiDB to read column-size statistics from `mysql.stats_histograms`. On clusters with a large number of tables, selecting any of these columns can consume more time and resources than selecting only `TABLE_ROWS`, which only requires row-count statistics from `mysql.stats_meta`. If you need only the estimated row count, select `TABLE_ROWS` without the size columns to avoid this overhead.


Most of the information in the table is the same as MySQL. The following columns are newly defined by TiDB:

* `TIDB_TABLE_ID`: to indicate the internal ID of a table. This ID is unique in a TiDB cluster.
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