TAIL

TAIL streams updates from a source, table, or view as they occur.

Conceptual framework

The TAIL statement is a more general form of a SELECT statement. While a SELECT statement computes a relation at a moment in time, a tail operation computes how a relation changes over time.

Fundamentally, TAIL 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 describes the complete set of changes to a relation, in order, while the TAIL is active.

Clients can use TAIL to:

• Power event processors that react to every change to a relation.
• Replicate the complete history of a relation while the TAIL is active.

Syntax

Supported WITH option values:

Details

Output

TAIL emits a sequence of updates. Each row contains all of the columns of the tailed relation, prepended with several additional columns that describe the nature of the update:

TAIL will continue to run until cancelled, or until all updates the tailed item could undergo have been presented. The latter case typically occurs when tailing constant views (e.g. CREATE VIEW v AS SELECT 1) or file sources that were created in non-tailing mode (tail = false).

Changed in v0.5.1: The timestamp and diff information moved to leading, well-typed columns. Previously the timestamp and diff information was encoded in a human-readable string in a trailing text column.

Changed in v0.5.1: TAIL sends rows to the client normally, i.e., as if they were sent by a SELECT statement. Previously TAIL was implicitly wrapped in a COPY TO statement.

Changed in v0.5.2: TAIL is now guaranteed to send timestamps in non-decreasing order.

Changed in v0.5.2: Syntax has changed. WITH SNAPSHOT is now WITH (SNAPSHOT). WITHOUT SNAPSHOT is now WITH (SNAPSHOT = false).

Changed in v0.5.2: The PROGRESS option has been added.

AS OF

The AS OF clause specifies the time at which a TAIL operation begins. See SNAPSHOT below for details on what this means.

If you don’t specify AS OF explicitly, Materialize will pick a timestamp automatically:

• If the tailed relation is materialized, Materialize picks the latest time for which results are computed.
• If the tailed relation is not materialized, Materialize picks time 0.

A given timestamp will be rejected if data it would report has already been compacted by Materialize. See the --logical-compaction-window command-line option for details on Materialize’s compaction policy.

SNAPSHOT

By default, a TAIL begins by emitting a snapshot of the tailed relation, which consists of a series of updates describing the contents of the relation at its AS OF timestamp. After the snapshot, TAIL emits further updates as they occur.

For updates in the snapshot, the timestamp field will be fast-forwarded to the AS OF timestamp. For example, TAIL ... AS OF 21 would present an insert that occured at time 15 as if it occurred at time 21.

To see only updates after the AS OF timestamp, specify WITH (SNAPSHOT = false).

PROGRESS

If the PROGRESS option is specified via WITH (PROGRESS), an additional progressed column appears in the output. It is false if there may be more rows with the same timestamp. It is true if no more timestamps will appear that are strictly less than the timestamp. All further columns after progressed will be NULL in the true case.

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 legimate period of no updates.

Not all timestamps that appear will have a corresponding progressed row. For example, the following is a valid sequence of updates:

Notice how Materialize did not emit explicit progress messages for timestamps 1, 2, or 3. 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

TAIL produces rows similar to a SELECT statement, except that TAIL 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.

Tailing with FETCH

The recommended way to use TAIL is with DECLARE and FETCH. These must be used within a transaction. This allows you to limit the number of rows and the time window of your requests. First, declare a TAIL cursor:

BEGIN;
DECLARE c CURSOR FOR TAIL t;

Now use FETCH in a loop to retrieve some number of rows within a time window:

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');

FETCH with Python and psycopg2

#!/usr/bin/env python3

import psycopg2
import sys

dsn = "postgresql://localhost:6875/materialize?sslmode=disable"
conn = psycopg2.connect(dsn)

with conn.cursor() as cur:
    cur.execute("DECLARE c CURSOR FOR TAIL v")
    while True:
        cur.execute("FETCH ALL c")
        for row in cur:
            print(row)

Streaming with Python and psycopg3

Although psycopg3 can function identically as the psycopg2 example above, it also has a stream feature where rows are not buffered and we can thus use TAIL directly.

#!/usr/bin/env python3

import psycopg3
import sys

dsn = "postgresql://localhost:6875/materialize?sslmode=disable"
conn = psycopg3.connect(dsn)

conn = psycopg3.connect(dsn)
with conn.cursor() as cur:
    for row in cur.stream("TAIL t"):
        print(row)

FETCH with C# and Npgsql

var txn = conn.BeginTransaction();
new NpgsqlCommand("DECLARE c CURSOR FOR TAIL t", conn, txn).ExecuteNonQuery();
while (true)
{
    using (var cmd = new NpgsqlCommand("FETCH ALL c", conn, txn))
    using (var reader  = cmd.ExecuteReader())
    {
        while (reader.Read())
        {
            // More columns available with reader[3], etc.
            Console.WriteLine("ts: " + reader[0] + ", diff: " + reader[1] + ", column 1: " + reader[2]);
        }
    }
}