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* Update of english version of descriprion of the table function `file`. * New syntax for ReplacingMergeTree. Some improvements in text. * Significantly change article about SummingMergeTree. Article is restructured, text is changed in many places of the document. New syntax for table creation is described. * Descriptions of AggregateFunction and AggregatingMergeTree are updated. Russian version. * New syntax for new syntax of CREATE TABLE * Added english docs on Aggregating, Replacing and SummingMergeTree. * CollapsingMergeTree docs. English version. * 1. Update of CollapsingMergeTree. 2. Minor changes in markup * Update aggregatefunction.md * Update aggregatefunction.md * Update aggregatefunction.md * Update aggregatingmergetree.md * GraphiteMergeTree docs update. New syntax for creation of Replicated* tables. Minor changes in *MergeTree tables creation syntax. * Markup fix * Markup and language fixes * Clarification in the CollapsingMergeTree article * DOCAPI-4821. Sync between ru and en versions of docs. * Fixed the ambiguity in geo functions description. * Example of JOIN in ru docs * Deleted misinforming example. * Fixed links to IN operators. * Updated the description of ALTER MODIFY. * [RU] Updated ALTER MODIFY description. * Fixed anchors. * DOCAPI-3818: The Family of Log engines. StripeLog. Tocs sync. * DOCAPI-3818: Edits after review by Ivan Blinkov.
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StripeLog
Engine belongs to the family of log engines. See the common properties of log engines and their differences in the Log Engine Family article.
Use this engine in scenarios, when you need to write many tables with the small amount of data (less than 1 million rows).
Creating a Table
CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
column1_name [type1] [DEFAULT|MATERIALIZED|ALIAS expr1],
column2_name [type2] [DEFAULT|MATERIALIZED|ALIAS expr2],
...
) ENGINE = StripeLog
See the detailed description of CREATE TABLE query.
Writing the Data
The StripeLog engine stores all the columns in one file. The Log and TinyLog engines store columns in separate files. For each INSERT query, ClickHouse appends data block to the end of a table file, writing columns one by one.
For each table ClickHouse writes two files:
data.bin— Data file.index.mrk— File with marks. Marks contain offsets for each column of each data block inserted.
The StripeLog engine does not support the ALTER UPDATE and ALTER DELETE operations.
Reading the Data
File with marks allows ClickHouse parallelize the reading of data. This means that SELECT query returns rows in an unpredictable order. Use the ORDER BY clause to sort rows.
Example of Use
Creating a table:
CREATE TABLE stripe_log_table
(
timestamp DateTime,
message_type String,
message String
)
ENGINE = StripeLog
Inserting data:
INSERT INTO stripe_log_table VALUES (now(),'REGULAR','The first regular message')
INSERT INTO stripe_log_table VALUES (now(),'REGULAR','The second regular message'),(now(),'WARNING','The first warning message')
We used two INSERT queries to create two data block inside the data.bin file.
When selecting data, ClickHouse uses multiple threads. Each thread reads the separate data block and returns resulting rows independently as it finished. It causes that the order of blocks of rows in the output does not match the order of the same blocks in the input in the most cases. For example:
SELECT * FROM stripe_log_table
┌───────────timestamp─┬─message_type─┬─message────────────────────┐
│ 2019-01-18 14:27:32 │ REGULAR │ The second regular message │
│ 2019-01-18 14:34:53 │ WARNING │ The first warning message │
└─────────────────────┴──────────────┴────────────────────────────┘
┌───────────timestamp─┬─message_type─┬─message───────────────────┐
│ 2019-01-18 14:23:43 │ REGULAR │ The first regular message │
└─────────────────────┴──────────────┴───────────────────────────┘
Sorting the results (ascending order by default):
SELECT * FROM stripe_log_table ORDER BY timestamp
┌───────────timestamp─┬─message_type─┬─message────────────────────┐
│ 2019-01-18 14:23:43 │ REGULAR │ The first regular message │
│ 2019-01-18 14:27:32 │ REGULAR │ The second regular message │
│ 2019-01-18 14:34:53 │ WARNING │ The first warning message │
└─────────────────────┴──────────────┴────────────────────────────┘