glean-description-lookup

name	glean-description-lookup
description	Use this skill when looking up field descriptions for Mozilla Glean telemetry tables (tables ending in _live or _stable, e.g. <app>_stable.<ping>_v1). Fetches descriptions from the Glean Dictionary (dictionary.telemetry.mozilla.org) using WebFetch with targeted field extraction — only the fields referenced in query.sql, never the full table schema.

Glean Description Lookup

Composable: Invoked by schema-enricher (Step 2, priority 4 when the upstream source table ends in _live or _stable) When to use: A source table ends in _live or _stable and you need field descriptions for schema.yaml generation or enrichment

⚠️ Scope — Only for _live and _stable Tables

This skill applies only to:

• Tables ending in _live (e.g. <app>_live.<ping>_v1)
• Tables ending in _stable (e.g. <app>_stable.<ping>_v1)

These tables are NOT in the /sql directory. They are live Glean ingestion tables managed outside bigquery-etl.

For all other tables (_derived, _bi, mozdata, etc.) — read schema.yaml from the local /sql directory instead.

Workflow

Step 1: Confirm table is _live or _stable

Check the source table referenced in query.sql:

grep -Ei "FROM|JOIN" query.sql | grep -oiE "[a-zA-Z0-9_-]+\.(.*_(live|stable))\.[a-zA-Z0-9_]+"

⚠️ Backtick limitation: This grep only matches unquoted identifiers. Backtick-quoted references (e.g. `project`.`dataset_stable`.`table`) will not appear in the output. If the grep returns nothing, either scan the query visually for _stable/_live dataset names, or skip ahead to Step 2 — the helper script handles all backtick quoting styles automatically.

If no _live or _stable table is found → do not use this skill. Use the local /sql directory.

Step 2: Extract fields used from that table in query.sql

Choose the right flag based on your query:

Query structure	Flag to use	Why
One Glean source table	--glean-only	Filters output to only Glean source tables
Multiple source tables, want fields from all Glean tables	--glean-only	Same — filters out non-Glean source tables
Multiple source tables, want fields from one specific Glean table	--table <app>_stable.<ping>_v1	Shows URL and suggested prompt for that table only; field extraction still covers the full query

Run the helper script and record its full output:

# Single Glean table or all Glean tables
python scripts/extract_query_fields.py query.sql --glean-only

# Specific Glean table (dataset.table format; project prefix is optional)
python scripts/extract_query_fields.py query.sql --table <app>_stable.<ping>_v1

Record:

Source table: <app>_stable.<ping>_v1
Fields used: [<field_1>, <field_2>, <field_3>, ...]

⚠️ Alias limitation: The script captures all dotted identifiers but cannot determine which ones are table or UNNEST alias references. After running the script, review the field list for two issues:

• Remove any entries whose first segment is a known alias (e.g. e.submission_timestamp where e aliases the _stable table, or ev.name where ev is an UNNEST alias) — these are false positives with the alias prefix intact.
• Add any Glean paths that were only accessed via alias in the query and therefore do not appear in the list. Use the full unaliased Glean path (e.g. submission_timestamp, events[].name).

To find aliases: search query.sql for FROM <table> AS <alias> and CROSS JOIN UNNEST(...) AS <alias> patterns, then compare those alias names against the first segment of each field in the output.

⚠️ Field cap: The script always extracts and prints the full field list. However, the SUGGESTED WebFetch PROMPT section is capped at --max-fields (default 25). If you see a truncation warning on stderr, either use the full FIELD REFERENCES list manually (after reviewing for alias false positives), or re-run with --max-fields 50 (or higher) and then review the expanded list for aliases before using it.

⚠️ Keyword filtering: The script automatically excludes SQL keywords and function names (e.g. select, extract, count) from the field list to reduce false positives. If a field appears absent from the output, verify it is not being silently filtered as a keyword — check query.sql directly.

Step 3: Map the BigQuery table name to a Glean Dictionary URL

Derivation rule (works for most apps):

1. Strip _live or _stable suffix from the BigQuery dataset name → this is the app_id (dictionary URL form, uses underscores): <app_id>_stable → <app_id>
2. Strip the _v<N> suffix from the BigQuery table name → this is the Glean table name: <table>_v1 → <table>

BigQuery dataset:  <app_id>_stable  or  <app_id>_live
BigQuery table:    <table>_v1

app_id (dictionary URL form, underscores):  <app_id>
Glean table name:                           <table>

URL:  https://dictionary.telemetry.mozilla.org/apps/<app_id>/tables/<table>

Consult references/app_id_map.md for known app mappings and alternate app_id variations. If your app is not listed, the derivation rule above works for all apps. To verify directly, visit https://dictionary.telemetry.mozilla.org/ and search for the app name — the URL contains the correct app_id.

⚠️ Hyphen vs underscore: The Glean Dictionary URL uses underscores (<app_id>). The probeinfo API (probeinfo.telemetry.mozilla.org) requires hyphens (<app-id>). If using the probeinfo API as a fallback, replace _ with - in the app_id.

Step 4: Determine fetch strategy from table type

Derive the table type from your BigQuery table name (e.g. baseline_v1 → baseline, metrics_v1 → metrics). The table below gives guidance on what to expect — but the actual strategy decision is made in Step 5 based on how many fields you extracted in Step 2 (≤ 25 → targeted, > 25 → sectional).

Table type	Typical field count	Strategy
events / metrics	100–500+	Targeted: request only fields from Step 2
crash	30–100	Sectional or targeted
baseline	30–60	Sectional: request by category (e.g. client_info.*)
newtab	20–80	Targeted or sectional depending on field count
first_session	10–30	Usually safe to request full schema
deletion_request	< 10	Safe to request full schema

Step 5: Fetch descriptions with targeted WebFetch

Use your cleaned field list from Step 2 (after alias review). Never copy the script's SUGGESTED WebFetch PROMPT directly — it uses the raw unreviewed field list and may include alias false positives.

Choose your strategy by field count:

• ≤ 25 fields → use the targeted prompt (single request)
• > 25 fields → use the sectional prompt (one request per category: client_info, metrics, ping_info, etc.)

Targeted prompt:

WebFetch:
URL: https://dictionary.telemetry.mozilla.org/apps/<app_id>/tables/<table>
Prompt: "Extract the name, BigQuery type, mode (NULLABLE/REPEATED), and description for
these fields only: <comma-separated cleaned list from Step 2>. Return as a list with one entry per field."

Sectional prompt (for tables with 25+ fields — repeat per category):

WebFetch:
URL: https://dictionary.telemetry.mozilla.org/apps/<app_id>/tables/<table>
Prompt: "Extract the name, BigQuery type, mode (NULLABLE/REPEATED), and description for
all fields in the <category> section (e.g. client_info, ping_info, metrics). Return as a list with one entry per field."

Categories are the top-level field groupings shown on the Glean Dictionary table page (e.g. client_info, ping_info, metadata, metrics). Visit the dictionary URL directly to see the structure for your table and identify which categories to request. For metrics tables with many metric types, you can also request by sub-type (e.g. metrics.string, metrics.counter).

Step 6: Map Glean types to BigQuery types

Use this mapping when writing descriptions to schema.yaml:

Glean type	BigQuery type	Notes
string / text / url	STRING
quantity / counter	INTEGER
boolean	BOOLEAN
datetime	TIMESTAMP
date	DATE
uuid / jwe	STRING
labeled_counter / labeled_boolean / labeled_string	RECORD	Nested key/value pairs
memory_distribution / timing_distribution / custom_distribution	RECORD	Has values, sum, count subfields
rate	RECORD	Has numerator, denominator subfields
object	STRING	Serialized JSON
event (extras)	RECORD	Nested
url_encoded	STRING
ARRAY of X	(X type) + mode: REPEATED	In schema.yaml: set mode: REPEATED; if X is a struct, set type: RECORD

Step 7: Return structured results

Produce a lookup table mapping each field to its type and description. This table is used to populate schema.yaml — the schema_path column shows where in the YAML nesting hierarchy each field lives, which is essential for correctly placing descriptions in nested RECORD fields.

| field                              | glean_type | bq_type   | mode     | schema_path                                              | description |
|-----------------------------------|------------|-----------|----------|----------------------------------------------------------|-------------|
| submission_timestamp               | datetime   | TIMESTAMP | NULLABLE | fields[submission_timestamp]                             | The time at which the ping was submitted to the ingestion endpoint. |
| client_info.client_id              | uuid       | STRING    | NULLABLE | fields[client_info] → fields[client_id]                  | A UUID identifying the client. |
| metrics.<type>.<metric_name>       | string     | STRING    | NULLABLE | fields[metrics] → fields[<type>] → fields[<metric_name>] | Description from Glean Dictionary. |

Error Handling

Problem	Action
404 from Glean Dictionary	Try in order: (1) check alternate app_id variations in references/app_id_map.md; (2) visit https://dictionary.telemetry.mozilla.org/ and search for the app name to get the exact URL; (3) fall back to probeinfo API with hyphens: https://probeinfo.telemetry.mozilla.org/glean/<hyphenated-app-id>/metrics.
Field not found in response	Diagnose in order: (1) It may be nested under a RECORD parent — re-request the parent category (e.g. request client_info if client_info.client_id is missing). (2) It may be an alias false positive from Step 2 — verify the field exists in the Glean Dictionary at all. (3) The field name may use hyphens in the Dictionary (rare) — try replacing underscores with hyphens. (4) The field may not exist in this specific table type; check the dictionary or ask the data owner.
Table not in Glean Dictionary	Fall back to ./bqetl query schema update to generate schema structure, then infer descriptions from field name and product context. As a secondary fallback, try https://probeinfo.telemetry.mozilla.org/glean/<hyphenated-app-id>/metrics — note this API uses hyphens in the app_id (e.g. <app-id>, not <app_id>).
_stable table has different schema than _live	Use the _stable URL — stable tables reflect the validated ping schema.

Integration with Other Skills

Skill	Relationship
schema-enricher	Invokes this skill in Step 2 (priority 4) when the upstream source table ends in _live or _stable — no local schema.yaml exists for those tables
column-description-finder	Complementary — use column-description-finder for descriptions in base schema YAML files; use this skill for descriptions from raw Glean tables not in base schemas

Quick Reference

Glean Dictionary base URL:

https://dictionary.telemetry.mozilla.org/apps/<app_id>/tables/<table_name>

(app_id uses underscores — strip _live/_stable from dataset name; see references/app_id_map.md for known mappings)

Script:

# Extract all Glean field references from a query
python scripts/extract_query_fields.py path/to/query.sql --glean-only

# Scope to a specific source table (useful when query joins multiple tables)
python scripts/extract_query_fields.py path/to/query.sql --table <app>_stable.<ping>_v1

# Increase field cap for the suggested WebFetch prompt (default: 25)
python scripts/extract_query_fields.py path/to/query.sql --glean-only --max-fields 50