"""The series index on plugin_metrics, built in the background after boot (#5490).

A read of one metric over a day (a rule's preview, query_metrics, a chart)
walks the (source, resource, metric, recorded_at) index; without the value in
the index, each sample is then fetched from its own heap page, interleaved
with every other series, so a cold day measured 18.3 s against 63 ms warm
(#5485). With `value` INCLUDEd the read is an index-only scan over adjacent
index pages.

The index is built with CREATE INDEX CONCURRENTLY from a task started after
the app is serving, never in a migration: a build over the live table takes
minutes on slow storage, and a migration would hold the boot with Steward
down for all of it (#5220). Ingest and pages keep working while it builds.
Once it is valid the index it replaces is dropped. Later boots find it
present and only check.
"""
from __future__ import annotations

import time

from sqlalchemy import text

SERIES_INDEX = "ix_plugin_metrics_series_value"
# The plain series index the covering one replaces (migration 0023).
REPLACES = "ix_plugin_metrics_module_resource_metric_recorded"

_STATE = text("""
    SELECT i.indisvalid FROM pg_class c JOIN pg_index i ON i.indexrelid = c.oid
    WHERE c.relname = :name
""")


async def ensure_series_index(engine, log) -> str:
    """Make sure the covering series index exists and is valid, then drop the
    one it replaces. Returns "present" when it already was, else "built".

    A build a restart interrupted leaves an INVALID index behind; that one is
    dropped and built again. Runs outside a transaction, as CONCURRENTLY
    requires."""
    async with engine.connect() as conn:
        conn = await conn.execution_options(isolation_level="AUTOCOMMIT")
        valid = (await conn.execute(_STATE, {"name": SERIES_INDEX})).scalar()
        outcome = "present"
        if valid is not True:
            if valid is False:
                log.warning("metric index: %s was left half-built; dropping it to build again",
                            SERIES_INDEX)
                await conn.execute(text(f"DROP INDEX CONCURRENTLY IF EXISTS {SERIES_INDEX}"))
            log.info("metric index: building %s in the background; pages and ingest carry on",
                     SERIES_INDEX)
            started = time.monotonic()
            await conn.execute(text(
                f"CREATE INDEX CONCURRENTLY IF NOT EXISTS {SERIES_INDEX} ON plugin_metrics "
                "(source_module, resource_name, metric_name, recorded_at) INCLUDE (value)"))
            log.info("metric index: built %s in %.0fs", SERIES_INDEX, time.monotonic() - started)
            outcome = "built"
        if (await conn.execute(_STATE, {"name": REPLACES})).scalar() is not None:
            await conn.execute(text(f"DROP INDEX CONCURRENTLY IF EXISTS {REPLACES}"))
            log.info("metric index: dropped %s, which %s replaces", REPLACES, SERIES_INDEX)
    return outcome