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The Lightweight UPDATE Statement

The lightweight UPDATE statement updates rows in a table [db.]table that match the expression filter_expr. It is called "lightweight update" to contrast it to the ALTER TABLE ... UPDATE query, which is a heavyweight process that rewrites entire columns in data parts. It is only available for the MergeTree table engine family.

The filter_expr must be of type UInt8. This query updates values of the specified columns to the values of the corresponding expressions in rows for which the filter_expr takes a non-zero value. Values are cast to the column type using the CAST operator. Updating columns used in the calculation of the primary or partition keys is not supported.

Examples​

Lightweight updates do not update data immediately​

Lightweight UPDATE is implemented using patch parts - a special kind of data part that contains only the updated columns and rows. A lightweight UPDATE creates patch parts but does not immediately modify the original data physically in storage. The process of updating is similar to a INSERT ... SELECT ... query but the UPDATE query waits until the patch part creation is completed before returning.

The updated values are:

  • Immediately visible in SELECT queries through patches application
  • Physically materialized only during subsequent merges and mutations
  • Automatically cleaned up once all active parts have the patches materialized

Lightweight updates requirements​

Lightweight updates are supported for MergeTree, ReplacingMergeTree, CollapsingMergeTree engines and their Replicated and Shared versions.

To use lightweight updates, materialization of _block_number and _block_offset columns must be enabled using table settings enable_block_number_column and enable_block_offset_column.

Lightweight deletes​

A lightweight DELETE query can be run as a lightweight UPDATE instead of a ALTER UPDATE mutation. The implementation of lightweight DELETE is controlled by setting lightweight_delete_mode.

Performance considerations​

Advantages of lightweight updates:

  • The latency of the update is comparable to the latency of the INSERT ... SELECT ... query
  • Only updated columns and values are written, not entire columns in data parts
  • No need to wait for currently running merges/mutations to complete, therefore the latency of an update is predictable
  • Parallel execution of lightweight updates is possible

Potential performance impacts:

  • Adds an overhead to SELECT queries that need to apply patches
  • Skipping indexes and projections are not used for data parts that have patches to be applied
  • Small updates which are too frequent may lead to a "too many parts" error. It is recommended to batch several updates into a single query, for example by putting ids for updates in a single IN clause in the WHERE clause
  • Lightweight updates are designed to update small amounts of rows (up to about 10% of the table). If you need to update a larger amount, it is recommended to use the ALTER TABLE ... UPDATE mutation

Concurrent operations​

Lightweight updates don't wait for currently running merges/mutations to complete unlike heavy mutations. The consistency of concurrent lightweight updates is controlled by settings update_sequential_consistency and update_parallel_mode.

Update permissions​

UPDATE requires the ALTER UPDATE privilege. To enable UPDATE statements on a specific table for a given user, run:

Details of the implementation​

Patch parts are the same as the regular parts, but contain only updated columns and several system columns:

  • _part - the name of the original part
  • _part_offset - the row number in the original part
  • _block_number - the block number of the row in the original part
  • _block_offset - the block offset of the row in the original part
  • _data_version - the data version of the updated data (block number allocated for the UPDATE query)

On average it gives about 40 bytes (uncompressed data) of overhead per updated row in the patch parts. System columns help to find rows in the original part which should be updated. System columns are related to the virtual columns in the original part, which are added for reading if patch parts should be applied. Patch parts are sorted by _part and _part_offset.

Patch parts belong to different partitions than the original part. The partition id of the patch part is patch-<hash of column names in patch part>-<original_partition_id>. Therefore patch parts with different columns are stored in different partitions. For example three updates SET x = 1 WHERE <cond>, SET y = 1 WHERE <cond> and SET x = 1, y = 1 WHERE <cond> will create three patch parts in three different partitions.

Patch parts can be merged among themselves to reduce the amount of applied patches on SELECT queries and reduce the overhead. Merging of patch parts uses the replacing merge algorithm with _data_version as a version column. Therefore patch parts always store the latest version for each updated row in the part.

Lightweight updates don't wait for currently running merges and mutations to finish and always use a current snapshot of data parts to execute an update and produce a patch part. Because of that there can be two cases of applying patch parts.

For example if we read part A, we need to apply patch part X:

  • if X contains part A itself. It happens if A was not participating in merge when UPDATE was executed.
  • if X contains part B and C, which are covered by part A. It happens if there was a merge (B, C) -> A running when UPDATE was executed.

For these two cases there are two ways to apply patch parts respectively:

  • Using merge by sorted columns _part, _part_offset.
  • Using join by _block_number, _block_offset columns.

The join mode is slower and requires more memory than the merge mode, but it is used less often.

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