collector.stat_statements.query_length does not truncate properly

[jsadowyj]

What did you do?
Enabled the following collector options:

 - collector.stat_statements.include_query
 - collector.stat_statements.query_length=4000

What did you expect to see?
The query label is truncated to 4000 characters (including comments).

What did you see instead? Under which circumstances?

pg_stat_statements_block_read_seconds_total{datname="<redacted>",queryid="-5016773285470984866",user="metabase_ro"} 0
pg_stat_statements_block_write_seconds_total{datname="<redacted>",queryid="-5016773285470984866",user="metabase_ro"} 0
pg_stat_statements_calls_total{datname="<redacted>",queryid="-5016773285470984866",user="metabase_ro"} 5
pg_stat_statements_query_id{query="-- Metabase:: userID: 1 queryType: MBQL queryHash: 54162c2fced7fe3932b12602637694179314c62f535f392cbd3cea208aec60d2\nSELECT \"public\".\"notifications\".\"id\" AS \"id\", (\"public\".\"notifications\".\"metadata\"#>> array[$1]::text[])::text AS \"metadata → body\", (\"public\".\"notifications\".\"metadata\"#>> array[$2]::text[])::text AS \"metadata → data\", (\"public\".\"notifications\".\"metadata\"#>> array[$3, $4]::text[])::text AS \"metadata → data → amount\", (\"public\".\"notifications\".\"metadata\"#>> array[$5, $6]::text[])::text AS \"metadata → data → platform_type\", (\"public\".\"notifications\".\"metadata\"#>> array[$7, $8]::text[])::text AS \"metadata → data → receiver\", (\"public\".\"notifications\".\"metadata\"#>> array[$9, $10, $11]::text[])::text AS \"metadata → data → receiver → avatar\", (\"public\".\"notifications\".\"metadata\"#>> array[$12, $13, $14]::text[])::text AS \"metadata → data → receiver → first_name\", (\"public\".\"notifications\".\"metadata\"#>> array[$15, $16, $17]::text[])::text AS \"metadata → data → receiver → id\", (\"public\".\"notifications\".\"metadata\"#>> array[$18, $19, $20]::text[])::text AS \"metadata → data → receiver → last_name\", (\"public\".\"notifications\".\"metadata\"#>> array[$21, $22, $23]::text[])::text AS \"metadata → data → receiver → username\", (\"public\".\"notifications\".\"metadata\"#>> array[$24, $25, $26]::text[])::text AS \"metadata → data → sender → first_name\", (\"public\".\"notifications\".\"metadata\"#>> array[$27, $28, $29]::text[])::text AS \"metadata → data → sender → id\", (\"public\".\"notifications\".\"metadata\"#>> array[$30, $31, $32]::text[])::text AS \"metadata → data → sender → last_name\", (\"public\".\"notifications\".\"metadata\"#>> array[$33, $34, $35]::text[])::text AS \"metadata → data → sender → username\", (\"public\".\"notifications\".\"metadata\"#>> array[$36, $37, $38]::text[])::text AS \"metadata → data → transaction → amount\", (\"public\".\"notifications\".\"metadata\"#>> array[$39, $40, $41]::text[])::text AS \"metadata → data → transaction → amountUsd\", (\"public\".\"notifications\".\"metadata\"#>> array[$42, $43, $44]::text[])::text AS \"metadata → data → transaction → blockExplorerLink\", (\"public\".\"notifications\".\"metadata\"#>> array[$45, $46, $47]::text[])::text AS \"metadata → data → transaction → from\", (\"public\".\"notifications\".\"metadata\"#>> array[$48, $49, $50]::text[])::text AS \"metadata → data → transaction → gasFee\", (\"public\".\"notifications\".\"metadata\"#>> array[$51, $52, $53, $54]::text[])::text AS \"metadata → data → transaction → original → error\", (\"public\".\"notifications\".\"metadata\"#>> array[$55, $56, $57]::text[])::text AS \"metadata → data → transaction → to\", (\"public\".\"notifications\".\"metadata\"#>> array[$58, $59, $60]::text[])::text AS \"metadata → data → transaction → transactionHash\", (\"public\".\"notifications\".\"metadata\"#>> array[$61, $62]::text[])::text AS \"metadata → data → transaction_hash\", (\"public\".\"notifications\".\"metadata\"#>> array[$63, $64]::text[])::text AS \"metadata → data → ulid\", (\"public\".\"notifications\".\"metadata\"#>> array[$65]::text[])::text AS \"metadata → event\", (\"public\".\"notifications\".\"metadata\"#>> array[$66]::text[])::text AS \"metadata → notification_type\", (\"public\".\"notifications\".\"metadata\"#>> array[$67, $68]::text[])::text AS \"metadata → payload → amount\", (\"public\".\"notifications\".\"metadata\"#>> array[$69, $70]::text[])::text AS \"metadata → payload → chainId\", (\"public\".\"notifications\".\"metadata\"#>> array[$71, $72]::text[])::bigint AS \"metadata → payload → creationHeight\", (\"public\".\"notifications\".\"metadata\"#>> array[$73, $74]::text[])::text AS \"metadata → payload → description\", (\"public\".\"notifications\".\"metadata\"#>> array[$75, $76]::text[])::text AS \"metadata → payload → from\", (\"public\".\"notifications\".\"metadata\"#>> array[$77, $78]::text[])::text AS \"metadata → payload → memo\", (\"public\".\"notifications\".\"metadata\"#>> array[$79, $80]::text[])::text AS \"metadata → payload → symbol\", (\"public\".\"notifications\".\"metadata\"#>> array[$81, $82]::text[])::text AS \"metadata → payload → to\", (\"public\".\"notifications\".\"metadata\"#>> ar",queryid="-5016773285470984866"} 1
pg_stat_statements_rows_total{datname="<redacted>",queryid="-5016773285470984866",user="metabase_ro"} 10000
pg_stat_statements_seconds_total{datname="<redacted>",queryid="-5016773285470984866",user="metabase_ro"} 42.436016087

The label length always exceeds 4000 bytes for the above query:

$ pbpaste | wc -c
    4451

Environment

• System information:

# uname -srm
Linux 6.8.0-35-generic x86_64

• postgres_exporter version:

# postgres_exporter --version
postgres_exporter, version 0.18.1 (branch: HEAD, revision: 320b684f3dcd3f09b6e694e43f9b389b1a8199e5)
  build user:       root@83cf77f7d3ab
  build date:       20250929-17:16:35
  go version:       go1.24.7
  platform:         linux/amd64
  tags:             unknown

• postgres_exporter flags:

usr/local/bin/postgres_exporter \
    --collector.stat_statements \
    --collector.stat_statements.include_query \
    --collector.stat_statements.query_length=4000 \
    --collector.replication \
    --collector.replication_slot \
    --collector.stat_wal_receiver \
    --web.listen-address=0.0.0.0:9187 \
    --config.file=/etc/postgres_exporter/postgres_exporter.yml \
    --web.telemetry-path=/metrics

• PostgreSQL version:

postgres=# select version();
                                                         version
--------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 15.10 - Percona Distribution on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 13.2.0-23ubuntu4) 13.2.0, 64-bit
(1 row)

• Logs:

None to speak of.

[eugercek]

Hi this happens because when query becomes label value, it needs to be escaped. Also your query has unicode characters and they count as multiple bytes with just wc. See below snippet.

# I copy pasted your query into a file called wc
> wc query
       0     361    4451 query

# This handles counting unicode as single character, for example `→`
>  wc -m query
    4287 query

# Prometheus needs to escape "foo" into \"foo\" because labels are defined as "...", 
> gsed 's/\\"/"/g' query | wc -m
    4001

---

We can add a function that handles this and make sure query length is under the threshold user give, it can be useful for ORMs that generate huge queries.

[eugercek]

I was making a PR for this, but I believe current behaviour is better. Because if I say I want first 1000 characters, I really want to see those 1000 characters. If we emit it because of the size in Prometehus label value, the query will be truncated just because of the size in Prometheus. As a down side, ORM generated queries will be much longer in label value in Prometheus. There are pros and cons of both solution.

I have a working patch, if we decide to go on the other side can submit it. Basically it has below, Branch Link

func truncateQuery(query string, maxLen int) string {
	count := 0
	for i, r := range query {
		runeLen := 1
		if r == '\\' || r == '"' || r == '\n' {
			runeLen = 2
		}
		if count+runeLen > maxLen {
			return query[:i]
		}
		count += runeLen
	}
	return query
}