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Parses SQL queries using a copy of the PostgreSQL server query parser


~> 3.0
= 0.49.1


 Project Readme


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 $n) 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 may seem like a lot of complexity, but is the only reliable way of parsing all valid PostgreSQL queries.

You can find further examples and a longer rationale here: https://pganalyze.com/blog/parse-postgresql-queries-in-ruby.html


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::ParserResult:0x000000012ec4e9e0
  @query="SELECT 1",
    version: 160001,
    stmts: [
        stmt: <PgQuery::Node:
          select_stmt: <PgQuery::SelectStmt:
            distinct_clause: [],
            target_list: [
                res_target: <PgQuery::ResTarget:
                  name: "",
                  indirection: [],
                  val: <PgQuery::Node:
                    a_const: <PgQuery::A_Const:
                      ival: <PgQuery::Integer: ival: 1>,
                      isnull: false,
                      location: 7
                  location: 7
            from_clause: [],
            group_clause: [],
            group_distinct: false,
            window_clause: [],
            values_lists: [],
            sort_clause: [],
            limit_option: :LIMIT_OPTION_DEFAULT,
            locking_clause: [],
            op: :SETOP_NONE,
            all: false
        stmt_location: 0,
        stmt_len: 0

Modifying a parsed query and turning it into SQL again

This is a simple example for deparse, for more complex modification, use walk!.

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

# Modify the parse tree in some way
parsed_query.tree.stmts[0].stmt.select_stmt.from_clause[0].range_var.relname = 'other_users'

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

Parsing a normalized query

# Normalizing a query (like pg_stat_statements in Postgres 10+)
PgQuery.normalize("SELECT 1 FROM x WHERE y = 'foo'")

=> "SELECT $1 FROM x WHERE y = $2"

Extracting tables from a query

PgQuery.parse("SELECT $1 FROM x JOIN y USING (id) WHERE z = $2").tables

=> ["x", "y"]

Extracting columns from a query

PgQuery.parse("SELECT $1 FROM x WHERE x.y = $2 AND z = $3").filter_columns

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

Fingerprinting a query

PgQuery.parse("SELECT 1").fingerprint

=> "50fde20626009aba"

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

=> "50fde20626009aba"

# Faster fingerprint method that is implemented inside the native C library
PgQuery.fingerprint("SELECT $1")

=> "50fde20626009aba"

Scanning a query into tokens

PgQuery.scan('SELECT 1 --comment')

=> [<PgQuery::ScanResult: version: 160001, tokens: [
<PgQuery::ScanToken: start: 0, end: 6, token: :SELECT, keyword_kind: :RESERVED_KEYWORD>,
<PgQuery::ScanToken: start: 7, end: 8, token: :ICONST, keyword_kind: :NO_KEYWORD>,
<PgQuery::ScanToken: start: 9, end: 18, token: :SQL_COMMENT, keyword_kind: :NO_KEYWORD>]>,

Walking the parse tree

For generalized use, PgQuery provides walk! as a means to recursively work with the parsed query.

This can be used to create a bespoke pretty printer:

parsed_query = PgQuery.parse "SELECT * FROM tbl"
parsed_query.walk! { |node, k, v, location| puts k }

More usefully, this can be used to rewrite a query. For example:

parsed_query.walk! do |node, k, v, location|
  next unless k.eql?(:range_var) || k.eql?(:relation)
  next if v.relname.nil?
  v.relname = "X_" + v.relname


There are some caveats, and limitations, in this example.

First, some of the tree nodes are frozen. You can replace them, but you cannot modify in place.

Second, table rewriting is a bit more nuanced than this example. While this will rewrite the table names, it will not correctly handle all CTEs, or rewrite columns with explicit table names.

Supported Ruby Versions

Currently tested and officially supported Ruby versions:

  • CRuby 2.6
  • CRuby 2.7
  • CRuby 3.0
  • CRuby 3.1
  • CRuby 3.2

Not supported:

  • JRuby: pg_query relies on a C extension, which is discouraged / not properly supported for JRuby
  • TruffleRuby: GraalVM does not support sigjmp, which is used by the Postgres error handling code (pg_query uses a copy of the Postgres parser & error handling code)

Developer tasks

Update libpg_query source

In order to update to a newer Postgres parser, first update libpg_query to the new Postgres version and tag a release.

Once that is done, follow the following steps:

  1. Update LIB_PG_QUERY_TAG and LIB_PG_QUERY_SHA256SUM in Rakefile

  2. Run rake update_source to update the source code

  3. Commit the Rakefile and the modified files in ext/pg_query to this source tree and make a PR


See libpg_query for pg_query in other languages, as well as products/tools built on pg_query.

Original Author

Special Thanks to


PostgreSQL server source code, used under the PostgreSQL license.
Portions Copyright (c) 1996-2023, The PostgreSQL Global Development Group
Portions Copyright (c) 1994, The Regents of the University of California

All other parts are licensed under the 3-clause BSD license, see LICENSE file for details.
Copyright (c) 2015, Lukas Fittl lukas@fittl.com
Copyright (c) 2016-2023, Duboce Labs, Inc. (pganalyze) team@pganalyze.com