-- 03_broken_migration.sql
-- THE BROKEN MIGRATION — do not run this on production.
-- This is the verbatim migration from the change request, with annotations.
-- It is intentionally left broken to demonstrate the failure modes.
--
-- Run in Session A. Watch Session B (monitoring) while this runs.
--
-- ISSUES DEMONSTRATED:
--   1. Single transaction wrapping the entire backfill → MVCC bloat
--   2. Partition range only covers FUTURE months → immediate INSERT failure
--   3. CREATE INDEX without CONCURRENTLY inside the transaction → ShareLock
--   4. No chunking → all-or-nothing, no progress on failure
--
-- EXPECTED OUTCOME:
--   The INSERT will fail with:
--     ERROR: no partition of relation "ci_job_artifacts" found for row
--   But before it fails, the open BEGIN will already be visible in:
--     demo.long_running_tx, demo.xmin_horizon
--   And dead tuples in demo.ci_build_status will be climbing because
--   autovacuum cannot advance past our snapshot.

-- ── DROP target if it exists from a previous run ─────────────────────────────
DROP TABLE IF EXISTS demo.ci_job_artifacts CASCADE;

BEGIN;

-- ── CREATE partitioned table ──────────────────────────────────────────────────
CREATE TABLE demo.ci_job_artifacts (
    id              BIGSERIAL,
    job_id          BIGINT    NOT NULL,
    project_id      BIGINT    NOT NULL,
    file_type       SMALLINT  NOT NULL DEFAULT 0,
    size_bytes      BIGINT,
    file_store      SMALLINT  NOT NULL DEFAULT 1,
    checksum        TEXT,
    created_at      TIMESTAMP NOT NULL DEFAULT NOW(),
    expire_at       TIMESTAMP,
    locked          BOOLEAN   DEFAULT FALSE,
    partition_key   DATE      NOT NULL DEFAULT CURRENT_DATE,
    PRIMARY KEY (id, partition_key)
) PARTITION BY RANGE (partition_key);

-- ISSUE 2: Creates partitions from CURRENT_DATE forward (future months only).
-- The backfill below selects rows from the PAST 6 months.
-- These rows have no matching partition → every INSERT row will error.
DO $$
DECLARE
    start_date      DATE := DATE_TRUNC('month', CURRENT_DATE);
    partition_name  TEXT;
    i               INT;
BEGIN
    FOR i IN 0..5 LOOP
        partition_name := 'ci_job_artifacts_' || TO_CHAR(start_date + (i || ' months')::INTERVAL, 'YYYY_MM');
        EXECUTE FORMAT(
            'CREATE TABLE demo.%I PARTITION OF demo.ci_job_artifacts
             FOR VALUES FROM (%L) TO (%L)',
            partition_name,
            start_date + (i || ' months')::INTERVAL,
            start_date + ((i + 1) || ' months')::INTERVAL
        );
    END LOOP;
END $$;

-- ISSUE 3: Indexes created inside the transaction, without CONCURRENTLY.
-- These hold ShareLock for the entire transaction duration.
CREATE INDEX idx_ci_job_artifacts_job_id      ON demo.ci_job_artifacts (job_id);
CREATE INDEX idx_ci_job_artifacts_project_id  ON demo.ci_job_artifacts (project_id);
CREATE INDEX idx_ci_job_artifacts_expire_at   ON demo.ci_job_artifacts (expire_at);
CREATE INDEX idx_ci_job_artifacts_checksum    ON demo.ci_job_artifacts (checksum);

-- ISSUE 1 + 2: Backfill inside a single transaction, targeting past 6 months.
-- Will fail because no partition exists for those months.
-- Even if we fix the partition range, running this as a single INSERT on
-- 5M rows (or 10TB at real scale) holds the transaction open for hours,
-- bloating MVCC dead tuples on every table touched by concurrent sessions.
INSERT INTO demo.ci_job_artifacts
SELECT
    id, job_id, project_id, file_type, size_bytes, file_store,
    checksum, created_at, expire_at, locked,
    DATE_TRUNC('month', created_at)::DATE AS partition_key
FROM demo.ci_job_artifacts_legacy
WHERE created_at >= NOW() - INTERVAL '6 months';

-- This COMMIT will never be reached due to the partition error above.
COMMIT;