SUBSCRIBE
SUBSCRIBE
streams updates from a source, table, or view as they occur.
Conceptual framework
The SUBSCRIBE
statement is a more general form of a SELECT
statement. While a SELECT
statement computes a relation at a moment in time, a
subscribe operation computes how a relation changes over time.
Fundamentally, SUBSCRIBE
produces a sequence of updates. An update describes either
the insertion or deletion of a row to the relation at a specific time. Taken
together, the updates describe the complete set of changes to a relation, in
order, while SUBSCRIBE
is active.
You can use SUBSCRIBE
to:
- Power event processors that react to every change to a relation or an arbitrary
SELECT
statement. - Replicate the complete history of a relation while
SUBSCRIBE
is active.
Syntax
Field | Use |
---|---|
object_name | The name of the source, table, or view that you want to subscribe to. |
select_stmt | The SELECT statement whose output you want to subscribe to. |
WITH
options
The following options are valid within the WITH
clause.
Option name | Value type | Default | Describes |
---|---|---|---|
SNAPSHOT |
boolean |
true |
Whether to emit a snapshot of the current state of the relation at the start of the operation. See SNAPSHOT below. |
PROGRESS |
boolean |
false |
Whether to include detailed progress information. See PROGRESS below. |
Details
Output
SUBSCRIBE
emits a sequence of updates as rows. Each row contains all of the
columns of the subscribed relation or derived from the SELECT
statement, prepended
with several additional columns that describe the nature of the update:
Column | Type | Represents |
---|---|---|
mz_timestamp |
numeric |
Materialize's internal logical timestamp. This will never be less than any
timestamp previously emitted by the same SUBSCRIBE operation.
|
mz_progressed |
boolean |
This column is only present if the
true , indicates that the SUBSCRIBE will not emit
additional records at times strictly less than mz_timestamp . See
PROGRESS below.
|
mz_diff |
bigint |
The change in frequency of the row. A positive number indicates that
mz_diff copies of the row were inserted, while a negative
number indicates that |mz_diff| copies of the row were deleted.
|
Column 1 | Varies | The columns from the subscribed relation, each as its own column, representing the data that was inserted into or deleted from the relation. |
... | ||
Column N | Varies |
AS OF
The AS OF
clause allows specifying a timestamp at which the SUBSCRIBE
should begin returning results, in order to inspect the historical state of a relation. If AS OF
is specified, no rows whose timestamp is less than the specified timestamp will be returned. If the timestamp specified is earlier than the earliest historical state retained by the source relations, an error will be signaled.
Currently, all user-defined sources and tables have a retention window of one second, so AS OF
is of limited usefulness except when subscribing to queries over certain internal relations.
UP TO
The UP TO
clause allows specifying a timestamp at which the SUBSCRIBE
will cease running. If UP TO
is specified, no rows whose timestamp is greater than or equal to the specified timestamp will be returned.
Interaction of AS OF
and UP TO
The lower timestamp bound specified by AS OF
is inclusive, whereas the upper bound specified by UP TO
is exclusive. Thus, a SUBSCRIBE
query whose AS OF
is equal to its UP TO
will terminate after returning zero rows.
A SUBSCRIBE
whose UP TO
is less than its “as of” timestamp (whether that timestamp was specified in an AS OF
clause or chosen by the system) will signal an error.
Duration
SUBSCRIBE
will continue to run until canceled, the session ends, the UP TO
timestamp is reached, or all updates have been presented. The latter case typically occurs when
tailing constant views (e.g. CREATE VIEW v AS SELECT 1
).
Many PostgreSQL drivers wait for a query to complete before returning its
results. Since SUBSCRIBE
can run forever, naively executing a SUBSCRIBE
using your
driver’s standard query API may never return.
Either use an API in your driver that does not buffer rows or use the
FETCH
statement or AS OF
and UP TO
bounds
to fetch rows from SUBSCRIBE
in batches.
See the examples for details.
SNAPSHOT
By default, SUBSCRIBE
begins by emitting a snapshot of the subscribed relation, which
consists of a series of updates describing the contents of the relation at its
initial timestamp. After the snapshot, SUBSCRIBE
emits further updates as
they occur.
For updates in the snapshot, the mz_timestamp
field will be fast-forwarded to the initial timestamp.
For example, an insert that occurred before the SUBSCRIBE
began would appear in the snapshot.
To see only updates after the initial timestamp, specify WITH (SNAPSHOT = false)
.
PROGRESS
Intuitively, progress messages communicate that no updates have occurred in a given time window. Without explicit progress messages, it is impossible to distinguish between a stall in Materialize and a legitimate period of no updates.
If the PROGRESS
option is specified via WITH (PROGRESS)
, an additional mz_progressed
column appears in the output.
When the column is false
, the rest of the row is a valid update.
When the column is true
the rest of the row is not a valid update and its content should be ignored;
the row exists only to communicate that timestamps have advanced.
Not all timestamps that appear will have a corresponding row with mz_progressed
set to true
.
For example, the following is a valid sequence of updates:
mz_timestamp | mz_progressed | mz_diff | column1
-------------|---------------|---------|--------------
1 | false | 1 | data
2 | false | 1 | more data
3 | false | 1 | even more data
4 | true | NULL | NULL
Notice how Materialize did not emit explicit progress messages for timestamps
1
or 2
. The receipt of the update at timestamp 2
implies that there
are no more updates for timestamp 1
, because timestamps are always presented
in non-decreasing order. The receipt of the explicit progress message at
timestamp 4
implies that there are no more updates for either timestamp
2
or 3
—but that there may be more data arriving at timestamp 4
.
Examples
SUBSCRIBE
produces rows similar to a SELECT
statement, except that SUBSCRIBE
may never complete.
Many drivers buffer all results until a query is complete, and so will never return.
Below are the recommended ways to work around this.
Creating a counter load generator
As an example, we’ll create a counter load generator that emits a row every second:
CREATE SOURCE counter
FROM LOAD GENERATOR COUNTER
WITH (SIZE = '3xsmall');
Subscribing with FETCH
The recommended way to use SUBSCRIBE
is with DECLARE
and FETCH
.
These must be used within a transaction, with a single DECLARE
per transaction.
This allows you to limit the number of rows and the time window of your requests.
Next, let’s subscribe to the counter
load generator source that we’ve created above.
First, declare a SUBSCRIBE
cursor:
BEGIN;
DECLARE c CURSOR FOR SUBSCRIBE (SELECT * FROM counter);
Then, use FETCH
in a loop to retrieve each batch of results as soon as it’s ready:
FETCH ALL c;
That will retrieve all of the rows that are currently available.
If there are no rows available, it will wait until there are some ready and return those.
A timeout
can be used to specify a window in which to wait for rows. This will return up to the specified count (or ALL
) of rows that are ready within the timeout. To retrieve up to 100 rows that are available in at most the next 1s
:
FETCH 100 c WITH (timeout='1s');
To retrieve all available rows available over the next 1s
:
FETCH ALL c WITH (timeout='1s');
A 0s
timeout can be used to return rows that are available now without waiting:
FETCH ALL c WITH (timeout='0s');
Using clients
If you want to use SUBSCRIBE
from an interactive SQL session (e.g.psql
), wrap the query in COPY
:
COPY (SUBSCRIBE (SELECT * FROM counter)) TO STDOUT;
Additional guides |
---|
Go |
Java |
Node.js |
PHP |
Python |
Ruby |
Mapping rows to their updates
After all the rows from the SNAPSHOT
have been transmitted, the updates will be emitted as they occur. How can you map each row to its corresponding update?
mz_timestamp | mz_progressed | mz_diff | Column 1 | …. | Column N | |
---|---|---|---|---|---|---|
1 | false | 1 | id1 | value1 | ||
1 | false | 1 | id2 | value2 | ||
1 | false | 1 | id3 | value3 | <- Last row from SNAPSHOT |
|
2 | false | -1 | id1 | value1 | ||
2 | false | 1 | id1 | value4 |
If your row has a unique column key, it is possible to map the update to its corresponding origin row; if the key is unknown, you can use the output of hash(columns_values)
instead.
In the example above, Column 1
acts as the column key that uniquely identifies the origin row the update refers to; in case this was unknown, hashing the values from Column 1
to Column N
would identify the origin row.
Dropping the counter
load generator source
When you’re done, you can drop the counter
load generator source:
DROP SOURCE counter;