cairnobs/docs/query-language-design.md

Query language design

Status: Design, approved 2026-08-14, not yet implemented (that's Task 3). This is the reference Task 3's implementation is built against — if implementation reveals this design is wrong somewhere, fix this doc in the same change, don't let them drift apart.

Why this design, in one paragraph

Phases 0–1 shipped two disconnected, placeholder query paths: raw SQL against ClickHouse, and free-text against Tantivy. Phase 2 needs one query language that can express both filter/aggregation and free-text search in a single query, without picking a winner between "give up structured querying" and "give up full-text search." The approach below does that by keeping parsing and execution strictly separate (a small pipe-syntax grammar and an "opaque SQL" passthrough both compile to the same IR) and by generalizing a mechanism Phase 1 already built and proved works (Tantivy-prefilter → ClickHouse IN (...)) rather than inventing a new cross-backend join strategy from scratch.

Grammar

Pipe syntax, SPL-inspired, EBNF-ish:

query        := base_search ("|" pipe_stage)*
base_search  := bool_expr                      // implicit filter/search, SPL convention
pipe_stage   := "where" bool_expr
              | "stats" agg_call ("," agg_call)* ["by" field ("," field)*]
              | "sort" sort_field ("," sort_field)*
              | "fields" field ("," field)*
              | "head" [INT]
              | "tail" [INT]

bool_expr    := term (("and" | "or") term)*
term         := field comparator value          // structured filter -> ClickHouse
              | "earliest" "=" time_expr        // time range lower bound
              | "latest" "=" time_expr          // time range upper bound
              | STRING | QUOTED_STRING          // bare term -> free-text (Tantivy) on `message`
              | "message" ":" QUOTED_STRING     // explicit free-text (Tantivy phrase/wildcard syntax passed through)

comparator   := "=" | "!=" | ">" | ">=" | "<" | "<="
agg_call     := IDENT "(" [field] ")" ["as" IDENT]   // count(), sum(field), avg(field), min(field), max(field)
sort_field   := ["-" | "+"] field                     // "-" = desc (default), "+" = asc
time_expr    := QUOTED_STRING                          // absolute RFC3339
              | "-" INT ("s"|"m"|"h"|"d"|"w")          // relative to query time, e.g. -1h, -7d
field        := IDENT

Worked examples

• service=api | where status>=500 | stats count by host | sort -count — service=api is the base filter (structured, top-level column); where status>=500 filters on status, which isn't a top-level column (see field mapping below); stats count by host aggregates; sort -count orders descending by the aggregate's implicit count alias.
• message:"connection refused" | stats count by host — free-text predicate feeding a ClickHouse aggregation. This is the case task 2 called "the hardest part" — see Execution below.
• SELECT host, count(*) FROM logs GROUP BY host — detected as SQL (see Detection below), executed directly against ClickHouse.

Parser: hand-written recursive descent, no new dependency

This grammar is small and stable — seven pipe-stage kinds, one expression grammar for filters. A hand-written lexer + recursive-descent parser beats a combinator library (e.g. participle) or a generator (goyacc) here:

• No new dependency. Consistent with "ask before adding a new external dependency, there's no case for one at this grammar size.
• Error messages matter for a user-facing query language in a way they don't for most internal parsing — "expected by after stats count, got sort" is easy to produce by hand, harder to get right through a combinator or generated parser.
• Generator tooling (goyacc) adds a codegen build step disproportionate to a grammar this size.
• This is the standard approach for small, real query DSLs at this scope — not a novel choice.

The SQL escape hatch: not parsed, wrapped as opaque IR

"Both syntaxes compile to the same IR" does not mean writing a SQL parser — reimplementing ClickHouse's SQL dialect would be a large, pointless undertaking when ClickHouse already parses its own SQL. A query that starts with SELECT (case-insensitive — see Detection) skips the pipe-syntax parser entirely and produces an IR value that wraps the raw SQL string as an opaque passthrough node. Both syntaxes still flow through the same Plan type and the same executor code path — that's what "same IR" actually buys (one execution and testing surface), not a shared abstract syntax tree. The existing SELECT-only / single-statement / keyword-blocklist validation (api/internal/queryapi/validate.go) is reused unchanged as the guard before wrapping.

IR (Plan)

type Plan struct {
    RawSQL      string            // set => everything else is ignored; opaque ClickHouse passthrough
    TextSearch  []TextPredicate   // bare terms / message: clauses -> routed to Tantivy
    Filters     []FilterPredicate // structured comparisons -> ClickHouse WHERE
    TimeRange   *TimeRange
    Aggregation *Aggregation      // nil => raw rows, no GROUP BY
    Sort        []SortField
    Fields      []string          // projection; empty => all columns
    Limit       *Limit            // head/tail
}

type TextPredicate struct {
    Query string // passed to Tantivy's query parser as-is
}

type FilterPredicate struct {
    Field string
    Op    string // "=", "!=", ">", ">=", "<", "<="
    Value string
}

type Aggregation struct {
    Funcs   []AggFunc // count/sum/avg/min/max, each with an optional field + alias
    GroupBy []string
}

type AggFunc struct {
    Func  string
    Field string // empty for count()
    Alias string
}

type SortField struct {
    Field string
    Desc  bool
}

type Limit struct {
    N    int
    Tail bool // true = last N (by time), false = first N
}

type TimeRange struct {
    From, To time.Time // relative expressions (-1h etc.) resolved to absolute at compile time
}

Field mapping: top-level columns vs. attributes

logs' real columns (per /storage) are timestamp, host, service, severity, message, attributes, record_id. Any field name in a query that isn't one of those maps to attributes['<field>'] — e.g. status>=500 compiles to a comparison against attributes['status'], not a top-level column, since status isn't promoted (Phase 1's decision not to promote anything without real usage data still holds). Because attributes is Map(String,String), every stored value is a string; numeric comparators against a non-top-level field cast via toFloat64OrZero(attributes['field']) when the compared value looks numeric, otherwise compare as string. This is what makes where status>=500 work against the existing schema with no migration — querying an unpromoted field is always slightly more expensive than a top-level column, which is worth knowing, not hiding.

Execution: routing between ClickHouse and Tantivy

The core mechanism already exists and is proven: Phase 1's /search endpoint (api/internal/queryapi/search.go, recordIDsQuery) already does exactly steps 1–2 below for text-only queries. Phase 2 generalizes it into four cases:

1. No TextSearch predicates → pure ClickHouse path. Build one SQL statement directly from Filters/TimeRange/Aggregation/Sort/ Fields/Limit. The common case, and the fast path.
2. TextSearch predicates, no Aggregation → Phase 1's /search behavior, generalized: Tantivy resolves matching record_ids, then SELECT ... WHERE record_id IN (...) for the rows, with Filters/ TimeRange/Sort/Fields/Limit folded into that same statement.
3. TextSearch predicates and Aggregation — the genuinely new case (message:"connection refused" | stats count by host): Tantivy resolves matching record_ids as a prefilter, not a join, then one ClickHouse statement does WHERE record_id IN (...) AND <Filters> GROUP BY <...>. Aggregation always happens in ClickHouse; Tantivy only ever narrows which rows are eligible before that.
4. RawSQL set → executed as-is against ClickHouse, no Tantivy involvement regardless of what the SQL contains. The escape hatch is opaque by design — no attempt to detect free-text intent inside raw SQL.

Known scaling limitation

Steps 2/3's record_id IN (...) approach breaks down if a text search matches a large number of rows — the IN clause is a literal, quoted UUID list embedded in the query string. Phase 2's mitigation: cap the Tantivy prefilter at 5,000 results. Tantivy's TopDocs already returns most-relevant-first, so the cap keeps the best matches rather than an arbitrary truncation, but it's a real limitation on result completeness for very broad text searches combined with aggregation. Documented in /docs/query-language-reference.md, not silently swallowed.

This number isn't a first-principles estimate — running the Phase 2 benchmark against a real 1M-row dataset (/docs/phase-2-runbook.md) caught the original 10,000 cap failing outright: 10,000 quoted UUIDs (~39 bytes each) produces a ~390KB query string, which exceeds ClickHouse's default max_query_size (262144 bytes / 256KiB) and fails with a syntax error rather than degrading gracefully — a much lower ceiling than "multi-million-entry" suggested before anyone had actually tried it. 5,000 UUIDs (~195KB) stays safely under that default with headroom. The real long-term fix (streaming record_id batches, ClickHouse-side text indexing, a different join strategy, or simply raising max_query_size server-side with matching memory sizing) is explicitly future work, out of scope for Phase 2.

Where this lives: api/internal/querylang/

Not a new top-level component. This subsystem always executes in-process within /api — it doesn't run standalone, doesn't get its own Docker image, and needs both connections /api already holds (the ClickHouse driver, the search gRPC client). A new top-level directory would imply a new deployable service, which this isn't.

api/internal/querylang/
  lexer/     tokenizer
  ast/       parsed pipe-syntax tree
  parser/    tokens -> ast (recursive descent)
  ir/        Plan and supporting types
  planner/   ast -> Plan (field-mapping rule, SQL-passthrough detection)
  executor/  Plan -> results (the four-case routing above)

Mirrors the existing internal/queryapi, internal/searchclient convention already in /api. Each layer is independently testable per task 3's requirement: "pipe syntax X compiles to IR Y" tests live in parser/planner against fixture ASTs/Plans, no backend needed; "IR Y executes correctly" tests live in executor against fakes for both the ClickHouse and search-client interfaces, same pattern already used throughout /ingest and /api.

/query endpoint: auto-detect, with an explicit override

Detection: a request body's query starting with SELECT (case-insensitive, same rule validateSelectOnly already applies) is SQL; otherwise pipe syntax. Covers the overwhelming common case with no extra field required. An optional "language": "sql" | "spl" field in the request body overrides detection, for the rare case a pipe query legitimately starts with the literal word "select" as a bare search term. Auto-detect-with-override matches the shape of other inference-with-explicit-override choices already made in this stack (e.g. severity hints winning over parsed values when present) — good default ergonomics, no silent ambiguity once a caller cares enough to be explicit.