Closes#171. The /api/query cost guardrail used to dry-run a synthetic
`SELECT * FROM <table>` for each registered remote-BQ row referenced
by the user SQL — which made BigQuery estimate a full table scan, with
column projection, predicate pushdown, and partition pruning all
disabled. Narrow queries on big partitioned/clustered tables (the
documented happy path for `agnes query --remote`) hit ~30,000×
over-estimates and got rejected with 400 `remote_scan_too_large` even
when BQ's own dry-run reported single-digit MB.
Pavel's report on #171 traced the root cause and proposed the fix:
rewrite the user SQL to BQ-native syntax and dry-run it as a single
job, exactly the way `bq query --dry_run` works.
Implementation:
- New helper _rewrite_user_sql_for_bq_dry_run rewrites bare registered
names (word-boundary, case-insensitive, longest-first to avoid prefix
collisions) + bq."<ds>"."<tbl>" forms to backticked
`<project>.<ds>.<tbl>` paths.
- _bq_quota_and_cap_guard runs ONE dry-run on the rewritten SQL. Cap
check uses the real estimate.
- Fallback path: if BQ rejects with bq_bad_request (e.g. DuckDB-only
syntax like ::INT casts), the guard falls back to the pre-fix
per-table SELECT * approach so non-portable queries still get a
(loose) cap estimate instead of fail-opening. Non-parse BQ errors
(forbidden, upstream) still propagate as 502.
- _bq_guardrail_inputs now also returns name_lookups so the rewriter
has the (registered_name, bucket, source_table) mapping it needs.
- Per-table breakdown is unavailable from a composite dry-run; total
bytes are pinned to dry_run_set[0] for the post-flight
record_bytes(sum(...)) call to keep returning the right total.
Tests (7 new, 3 existing still pass):
- dry-run receives rewritten user SQL with WHERE clause intact (the
load-bearing assertion for #171)
- single dry-run per request even with multiple registered tables
(JOIN, UNION) referenced
- fallback to per-table SELECT * on bq_bad_request
- non-parse BQ errors (forbidden) still 502
- rewriter unit tests: bare + bq.path in same SQL, longest-name-wins
on prefix collision, case-insensitive bare-name match
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