This Ruby extension uses the actual PostgreSQL server source to parse SQL queries and return the internal PostgreSQL parsetree.

In addition the extension allows you to normalize queries (replacing constant values with ?) and parse these normalized queries into a parsetree again.

When you build this extension, it builds parts of the PostgreSQL server source (see libpg_query), and then statically links it into this extension.

This is slightly crazy, but is the only reliable way of parsing all valid PostgreSQL queries.

You can find further examples and a longer rationale here:


gem install pg_query

Due to compiling parts of PostgreSQL, installation might take a while on slower systems. Expect up to 5 minutes.


Parsing a query

PgQuery.parse("SELECT 1")

=> #<PgQuery:0x007fe92b27ea18
          {"val"=>{"A_Const"=>{"val"=>{"Integer"=>{"ival"=>1}}, "location"=>7}},
 @query="SELECT 1",

Modifying a parsed query and turning it into SQL again

parsed_query = PgQuery.parse("SELECT * FROM users")

=> #<PgQuery:0x007ff3e956c8b0
          {"ColumnRef"=> {"fields"=>[{"A_Star"=>{}}], "location"=>7}},
 @query="SELECT * FROM users",

# Modify the parse tree in some way
parsed_query.tree[0]['SelectStmt']['fromClause'][0]['RangeVar']['relname'] = 'other_users'

# Turn it into SQL again
=> "SELECT * FROM \"other_users\""

Note: The deparsing feature is experimental and does not support outputting all SQL yet.

Parsing a normalized query

# Normalizing a query (like pg_stat_statements)
PgQuery.normalize("SELECT 1 FROM x WHERE y = 'foo'")

=> "SELECT ? FROM x WHERE y = ?"

# Parsing a normalized query
PgQuery.parse("SELECT ? FROM x WHERE y = ?")

=> #<PgQuery:0x007fb99455a438
          "lexpr"=>{"ColumnRef"=>{"fields"=>[{"String"=>{"str"=>"y"}}], "location"=>22}},
 @query="SELECT ? FROM x WHERE y = ?",

Extracting tables from a query

PgQuery.parse("SELECT ? FROM x JOIN y USING (id) WHERE z = ?").tables

=> ["x", "y"]

Extracting columns from a query

PgQuery.parse("SELECT ? FROM x WHERE x.y = ? AND z = ?").filter_columns

=> [["x", "y"], [nil, "z"]]

Fingerprinting a query

PgQuery.parse("SELECT 1").fingerprint

=> "8e1acac181c6d28f4a923392cf1c4eda49ee4cd2"

PgQuery.parse("SELECT 2; --- comment").fingerprint

=> "8e1acac181c6d28f4a923392cf1c4eda49ee4cd2"

# Faster fingerprint method that is implemented inside the native library
PgQuery.fingerprint("SELECT ?")

=> "8e1acac181c6d28f4a923392cf1c4eda49ee4cd2"

Differences from Upstream PostgreSQL

This gem is based on libpg_query, which uses the latest stable PostgreSQL version, but with a patch applied to support parsing normalized queries containing ? replacement characters.

Supported Ruby Versions

Currently tested and officially supported Ruby versions:

  • MRI 1.9
  • MRI 2.0
  • MRI 2.1
  • MRI 2.2
  • MRI 2.3
  • Rubinius 3.20


Products, tools and libraries built on pg_query:

pg_query for other languages:

Please feel free to open a PR to add yours! :)

Original Author

Special Thanks to


Copyright (c) 2015, pganalyze Team [email protected]
pg_query is licensed under the 3-clause BSD license, see LICENSE file for details.

Query normalization code:
Copyright (c) 2008-2015, PostgreSQL Global Development Group