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201 lines
7.6 KiB
Markdown
201 lines
7.6 KiB
Markdown
---
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slug: /en/sql-reference/statements/alter/view
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sidebar_position: 50
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sidebar_label: VIEW
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---
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# ALTER TABLE ... MODIFY QUERY Statement
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You can modify `SELECT` query that was specified when a [materialized view](../create/view.md#materialized) was created with the `ALTER TABLE ... MODIFY QUERY` statement without interrupting ingestion process.
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This command is created to change materialized view created with `TO [db.]name` clause. It does not change the structure of the underlying storage table and it does not change the columns' definition of the materialized view, because of this the application of this command is very limited for materialized views are created without `TO [db.]name` clause.
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**Example with TO table**
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```sql
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CREATE TABLE events (ts DateTime, event_type String)
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ENGINE = MergeTree ORDER BY (event_type, ts);
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CREATE TABLE events_by_day (ts DateTime, event_type String, events_cnt UInt64)
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ENGINE = SummingMergeTree ORDER BY (event_type, ts);
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CREATE MATERIALIZED VIEW mv TO events_by_day AS
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SELECT toStartOfDay(ts) ts, event_type, count() events_cnt
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FROM events
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GROUP BY ts, event_type;
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INSERT INTO events
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SELECT Date '2020-01-01' + interval number * 900 second,
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['imp', 'click'][number%2+1]
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FROM numbers(100);
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SELECT ts, event_type, sum(events_cnt)
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FROM events_by_day
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GROUP BY ts, event_type
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ORDER BY ts, event_type;
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┌──────────────────ts─┬─event_type─┬─sum(events_cnt)─┐
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│ 2020-01-01 00:00:00 │ click │ 48 │
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│ 2020-01-01 00:00:00 │ imp │ 48 │
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│ 2020-01-02 00:00:00 │ click │ 2 │
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│ 2020-01-02 00:00:00 │ imp │ 2 │
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└─────────────────────┴────────────┴─────────────────┘
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-- Let's add the new measurment `cost`
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-- and the new dimension `browser`.
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ALTER TABLE events
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ADD COLUMN browser String,
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ADD COLUMN cost Float64;
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-- Column do not have to match in a materialized view and TO
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-- (destination table), so the next alter does not break insertion.
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ALTER TABLE events_by_day
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ADD COLUMN cost Float64,
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ADD COLUMN browser String after event_type,
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MODIFY ORDER BY (event_type, ts, browser);
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INSERT INTO events
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SELECT Date '2020-01-02' + interval number * 900 second,
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['imp', 'click'][number%2+1],
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['firefox', 'safary', 'chrome'][number%3+1],
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10/(number+1)%33
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FROM numbers(100);
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-- New columns `browser` and `cost` are empty because we did not change Materialized View yet.
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SELECT ts, event_type, browser, sum(events_cnt) events_cnt, round(sum(cost),2) cost
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FROM events_by_day
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GROUP BY ts, event_type, browser
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ORDER BY ts, event_type;
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┌──────────────────ts─┬─event_type─┬─browser─┬─events_cnt─┬─cost─┐
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│ 2020-01-01 00:00:00 │ click │ │ 48 │ 0 │
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│ 2020-01-01 00:00:00 │ imp │ │ 48 │ 0 │
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│ 2020-01-02 00:00:00 │ click │ │ 50 │ 0 │
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│ 2020-01-02 00:00:00 │ imp │ │ 50 │ 0 │
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│ 2020-01-03 00:00:00 │ click │ │ 2 │ 0 │
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│ 2020-01-03 00:00:00 │ imp │ │ 2 │ 0 │
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└─────────────────────┴────────────┴─────────┴────────────┴──────┘
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ALTER TABLE mv MODIFY QUERY
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SELECT toStartOfDay(ts) ts, event_type, browser,
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count() events_cnt,
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sum(cost) cost
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FROM events
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GROUP BY ts, event_type, browser;
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INSERT INTO events
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SELECT Date '2020-01-03' + interval number * 900 second,
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['imp', 'click'][number%2+1],
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['firefox', 'safary', 'chrome'][number%3+1],
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10/(number+1)%33
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FROM numbers(100);
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SELECT ts, event_type, browser, sum(events_cnt) events_cnt, round(sum(cost),2) cost
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FROM events_by_day
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GROUP BY ts, event_type, browser
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ORDER BY ts, event_type;
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┌──────────────────ts─┬─event_type─┬─browser─┬─events_cnt─┬──cost─┐
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│ 2020-01-01 00:00:00 │ click │ │ 48 │ 0 │
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│ 2020-01-01 00:00:00 │ imp │ │ 48 │ 0 │
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│ 2020-01-02 00:00:00 │ click │ │ 50 │ 0 │
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│ 2020-01-02 00:00:00 │ imp │ │ 50 │ 0 │
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│ 2020-01-03 00:00:00 │ click │ firefox │ 16 │ 6.84 │
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│ 2020-01-03 00:00:00 │ click │ │ 2 │ 0 │
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│ 2020-01-03 00:00:00 │ click │ safary │ 16 │ 9.82 │
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│ 2020-01-03 00:00:00 │ click │ chrome │ 16 │ 5.63 │
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│ 2020-01-03 00:00:00 │ imp │ │ 2 │ 0 │
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│ 2020-01-03 00:00:00 │ imp │ firefox │ 16 │ 15.14 │
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│ 2020-01-03 00:00:00 │ imp │ safary │ 16 │ 6.14 │
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│ 2020-01-03 00:00:00 │ imp │ chrome │ 16 │ 7.89 │
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│ 2020-01-04 00:00:00 │ click │ safary │ 1 │ 0.1 │
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│ 2020-01-04 00:00:00 │ click │ firefox │ 1 │ 0.1 │
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│ 2020-01-04 00:00:00 │ imp │ firefox │ 1 │ 0.1 │
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│ 2020-01-04 00:00:00 │ imp │ chrome │ 1 │ 0.1 │
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└─────────────────────┴────────────┴─────────┴────────────┴───────┘
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-- !!! During `MODIFY ORDER BY` PRIMARY KEY was implicitly introduced.
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SHOW CREATE TABLE events_by_day FORMAT TSVRaw
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CREATE TABLE test.events_by_day
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(
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`ts` DateTime,
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`event_type` String,
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`browser` String,
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`events_cnt` UInt64,
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`cost` Float64
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)
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ENGINE = SummingMergeTree
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PRIMARY KEY (event_type, ts)
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ORDER BY (event_type, ts, browser)
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SETTINGS index_granularity = 8192
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-- !!! The columns' definition is unchanged but it does not matter, we are not querying
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-- MATERIALIZED VIEW, we are querying TO (storage) table.
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-- SELECT section is updated.
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SHOW CREATE TABLE mv FORMAT TSVRaw;
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CREATE MATERIALIZED VIEW test.mv TO test.events_by_day
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(
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`ts` DateTime,
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`event_type` String,
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`events_cnt` UInt64
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) AS
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SELECT
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toStartOfDay(ts) AS ts,
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event_type,
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browser,
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count() AS events_cnt,
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sum(cost) AS cost
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FROM test.events
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GROUP BY
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ts,
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event_type,
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browser
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```
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**Example without TO table**
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The application is very limited because you can only change the `SELECT` section without adding new columns.
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```sql
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CREATE TABLE src_table (`a` UInt32) ENGINE = MergeTree ORDER BY a;
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CREATE MATERIALIZED VIEW mv (`a` UInt32) ENGINE = MergeTree ORDER BY a AS SELECT a FROM src_table;
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INSERT INTO src_table (a) VALUES (1), (2);
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SELECT * FROM mv;
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```
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```text
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┌─a─┐
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│ 1 │
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│ 2 │
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└───┘
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```
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```sql
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ALTER TABLE mv MODIFY QUERY SELECT a * 2 as a FROM src_table;
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INSERT INTO src_table (a) VALUES (3), (4);
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SELECT * FROM mv;
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```
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```text
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┌─a─┐
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│ 6 │
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│ 8 │
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└───┘
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┌─a─┐
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│ 1 │
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│ 2 │
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└───┘
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```
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## ALTER LIVE VIEW Statement
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`ALTER LIVE VIEW ... REFRESH` statement refreshes a [Live view](../create/view.md#live-view). See [Force Live View Refresh](../create/view.md#live-view-alter-refresh).
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## ALTER TABLE ... MODIFY REFRESH Statement
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`ALTER TABLE ... MODIFY REFRESH` statement changes refresh parameters of a [Refreshable Materialized View](../create/view.md#refreshable-materialized-view). See [Changing Refresh Parameters](../create/view.md#changing-refresh-parameters).
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