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Originally from: tweet, LinkedIn post.

How to use lib_pgquery in shell to normalize and match queries from various sources

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Why use lib_pgquery

In Day 12: How to find query examples for problematic pg_stat_statements records it was mentioned that query normalization can be done using lib_pgquery. This library builds a tree representation of query text, and also computes so-called "fingerprint" – a hash of the normalized form of the query (query text where all parameters are removed).

This is helpful in various cases, for example:

  • If you need to match normalized queries in pg_stat_statements with individual query texts from pg_stat_activity or Postgres logs, and you use Postgres version older than 14, where compute_query_id was implemented to solve this problem.
  • If you use newer version of Postgres, but compute_query_id is off.
  • If you use query texts from different sources and/or are unsure that the standard query_id (aka "queryid" - the naming is not unified across tables) can be a reliable way of matching.

The basic lib_pgquery is written in C, and it is used in libraries for various languages:

Docker version for CLI use

For convenience, my colleagues took the Go version and wrapped it into docker, allowing its use in CLI style, in shell:

❯ docker run --rm postgresai/pg-query-normalize \
    'select c1 from t1 where c2 = 123;'
{
  "query": "select c1 from t1 where c2 = 123;",
  "normalizedQuery": "select c1 from t1 where c2 = $1;",
  "fingerprint": "0212acd45326d8972d886d4b3669a90be9dd4a9853",
  "tree": [...]
  ]
}

– it produces a JSON.

To get the normalized query value, we can use jq:

❯ docker run --rm postgresai/pg-query-normalize \
      'select c1 from t1 where c2 = 123;' \
    | jq -r '.normalizedQuery'
select c1 from t1 where c2 = $1;

To extract just the fingerprint as text:

❯ docker run --rm postgresai/pg-query-normalize \
    'select c1 from t1 where c2 = 123;' \
  | jq -r '.fingerprint'
0212acd45326d8972d886d4b3669a90be9dd4a9853

If we use different parameters, fingerprint doesn't change – let's use 0 instead of 123:

❯ docker run --rm postgresai/pg-query-normalize \
    'select c1 from t1 where c2 = 0;' \
  | jq -r '.fingerprint'
0212acd45326d8972d886d4b3669a90be9dd4a9853

Normalized queries as input

And – this is a very good property! – if a query is already normalized, and we have placeholders ($1, $2, etc.) instead of parameters, the value remains the same:

❯ docker run --rm postgresai/pg-query-normalize \
    'select c1 from t1 where c2 = $1;' \
  | jq -r '.fingerprint'
0212acd45326d8972d886d4b3669a90be9dd4a9853

But it changes if the query is different – say, the SELECT clause is different, here we use select * instead of select c1, resulting in a new fingerprint value:

❯ docker run --rm postgresai/pg-query-normalize \
    'select * from t1 where c2 = $1;' \
  | jq -r '.fingerprint'
0293106c74feb862c398e267f188f071ffe85a30dd

IN lists

Finally, unlike the in-core queryid, lib_pgquery ignores the number of parameters in the IN clause – this behavior suits better for query normalization and matching. Compare:

❯ docker run --rm postgresai/pg-query-normalize \
    'select * from t1 where c2 in (1, 2, 3);' \
  | jq -r '.fingerprint'
022fad3ad8fab1b289076f4cfd6ef0a21a15d01386

❯ docker run --rm postgresai/pg-query-normalize \
  'select * from t1 where c2 in (1000);' \
| jq -r '.fingerprint'
022fad3ad8fab1b289076f4cfd6ef0a21a15d01386