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system tables: add a note for DATA_LENGTH, INDEX_LENGTH, and TABLE_SIZE #19391

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system tables: add a note for DATA_LENGTH, INDEX_LENGTH, and TABLE_SIZE #19391

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2 changes: 1 addition & 1 deletion information-schema/information-schema-table-storage-stats.md
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Expand Up @@ -59,5 +59,5 @@ Fields in the `TABLE_STORAGE_STATS` table are described as follows:
* `PEER_COUNT`: The number of replicas of the table.
* `REGION_COUNT`: The number of Regions.
* `EMPTY_REGION_COUNT`: The number of Regions that do not contain data in this table.
* `TABLE_SIZE`: The total size of the table, in the unit of MiB.
* `TABLE_SIZE`: The total size of the table, in the unit of MiB. Note that this value is a logical estimate based on table statistics and does not represent the actual compressed physical size in storage.
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The fact that this is based on statistics is good. However that it doesn't represent the "actual compressed physical size in storage" is not because the statistics aren't precise enough. This is because the size in storage and is measuring something different.

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So it represent the statistical size of the uncompressed data? With or without possible changed rows or tombstones?

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@qiancai qiancai Feb 7, 2025 •
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@dveeden How about the following changes?

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* `TABLE_SIZE`: The total size of the table, in the unit of MiB. Note that this value is a logical estimate based on table statistics and does not represent the actual compressed physical size in storage.
* `TABLE_SIZE`: The total size of the table, in the unit of MiB. Note that this value is a logical estimate based on table statistics, whereas the actual compressed physical size in storage depends on storage formats, compression algorithms, and other physical storage optimizations.

* `TABLE_KEYS`: The total number of records in the table.
4 changes: 2 additions & 2 deletions information-schema/information-schema-tables.md
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Expand Up @@ -102,9 +102,9 @@ The description of columns in the `TABLES` table is as follows:
* `ROW_FORMAT`: The row format. The value is currently `Compact`.
* `TABLE_ROWS`: The number of rows in the table in statistics.
* `AVG_ROW_LENGTH`: The average row length of the table. `AVG_ROW_LENGTH` = `DATA_LENGTH` / `TABLE_ROWS`.
* `DATA_LENGTH`: Data length. `DATA_LENGTH` = `TABLE_ROWS` \* the sum of storage lengths of the columns in the tuple. The replicas of TiKV are not taken into account.
* `DATA_LENGTH`: Data length. `DATA_LENGTH` = `TABLE_ROWS` \* the sum of storage lengths of the columns in the tuple. The replicas of TiKV are not taken into account. Note that this value is a logical estimate based on table statistics and does not represent the actual compressed physical size in storage.
* `MAX_DATA_LENGTH`: The maximum data length. The value is currently `0`, which means the data length has no upper limit.
* `INDEX_LENGTH`: The index length. `INDEX_LENGTH` = `TABLE_ROWS` \* the sum of lengths of the columns in the index tuple. The replicas of TiKV are not taken into account.
* `INDEX_LENGTH`: The index length. `INDEX_LENGTH` = `TABLE_ROWS` \* the sum of lengths of the columns in the index tuple. The replicas of TiKV are not taken into account. Note that this value is a logical estimate based on table statistics and does not represent the actual compressed physical size in storage.
* `DATA_FREE`: Data fragment. The value is currently `0`.
* `AUTO_INCREMENT`: The current step of the auto- increment primary key.
* `CREATE_TIME`: The time at which the table is created.
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