Discussion: create source without consuming data until a start command

[chenzl25]

Currently, when users create a source and then create some Mvs on it, it will immediately consume data. However, in some circumstances, it will be better if we don't consume it first.

Case1: Users may want to rebuild the entire streaming job without waiting for each individual Mv to be backfilled. To achieve this, they can create a source without consuming the data. Once the pipeline is built, they can then start consuming the source data.

Case2: I have found that a user used temporal join to enforce some invariants for the output data. To ensure that the invariants are valid for all output, we need to ensure that there is no data consumption between Mv latest_b_per_kind and a_b. Otherwise, the temporal join may lookup future data.

SET streaming_parallelism = 1;
​
CREATE TABLE events (seq bigint, event_type int,  kind varchar) APPEND ONLY
WITH (
    connector = 'datagen',
    fields.seq.kind = 'sequence',
    fields.seq.end = '9223372036854775807',
    fields.event_type.kind = 'random',
    fields.event_type.min = '1',
    fields.event_type.max = '2',
    fields.event_type.seed = 1,
    fields.kind.kind = 'random',
    fields.kind.length = 2,
    fields.kind.seed = 1,
    datagen.rows.per.second = 10000
) FORMAT PLAIN ENCODE JSON;
​
​
CREATE MATERIALIZED VIEW IF NOT EXISTS a AS (
    SELECT
        DISTINCT ON (seq) seq,
        kind
    FROM
        events
    WHERE
        event_type = 1
);
​
CREATE MATERIALIZED VIEW IF NOT EXISTS b AS (
    SELECT
        DISTINCT ON (seq) seq,
        kind
    FROM
        events
    WHERE
        event_type = 2
);
​
CREATE MATERIALIZED VIEW IF NOT EXISTS latest_b_per_kind AS (
    SELECT
        kind,
        seq
    FROM
        (
            SELECT
                kind,
                seq,
                row_number() OVER (
                    PARTITION BY kind
                    ORDER BY
                        seq DESC
                ) as rank
            FROM
                b
        )
    WHERE
        rank = 1
);
​
CREATE MATERIALIZED VIEW IF NOT EXISTS a_b AS (
    SELECT
        a.seq as a_seq,
        a.kind as kind,
        latest_b_per_kind.seq as b_seq
    FROM
        a
        LEFT JOIN latest_b_per_kind FOR SYSTEM_TIME AS OF PROCTIME() ON a.kind = latest_b_per_kind.kind
);

Check invariant:

SELECT count(1) from a_b where b_seq is not null and a_seq < b_seq;

[hzxa21]

This can also be a very useful admin operations for troubleshooting and issue mitigation. Related: https://github.com/risingwavelabs/risingwave/issues/12997

[BugenZhao]

There's a similar issue long ago: https://github.com/risingwavelabs/risingwave/issues/3073

[StrikeW]

For case1, I think transactional DDL makes more sense.

[st1page]

some discussion about case1: https://github.com/risingwavelabs/risingwave/issues/12771

[github-actions[bot]]

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