DuckDB's SQL dialect closely follows the conventions of the PostgreSQL dialect. The few exceptions to this are listed on this page.

Floating-Point Arithmetic

DuckDB and PostgreSQL handle floating-point arithmetic differently for division by zero. DuckDB conforms to the IEEE Standard for Floating-Point Arithmetic (IEEE 754) for both division by zero and operations involving infinity values. PostgreSQL returns an error for division by zero but aligns with IEEE 754 for handling infinity values. To show the differences, run the following SQL queries:

SELECT 1.0 / 0.0 AS x;
SELECT 0.0 / 0.0 AS x;
SELECT -1.0 / 0.0 AS x;
SELECT 'Infinity'::FLOAT / 'Infinity'::FLOAT AS x;
SELECT 1.0 / 'Infinity'::FLOAT AS x;
SELECT 'Infinity'::FLOAT - 'Infinity'::FLOAT AS x;
SELECT 'Infinity'::FLOAT - 1.0 AS x;

Division on Integers

When computing division on integers, PostgreSQL performs integer division, while DuckDB performs float division:

SELECT 1 / 2 AS x;

PostgreSQL returns 0, while DuckDB returns 0.5.

To perform integer division in DuckDB, use the // operator:

SELECT 1 // 2 AS x;

This returns 0.

UNION of Boolean and Integer Values

The following query fails in PostgreSQL but successfully completes in DuckDB:

SELECT true AS x
UNION
SELECT 2;

PostgreSQL returns an error:

ERROR:  UNION types boolean and integer cannot be matched

DuckDB performs an enforced cast, therefore, it completes the query and returns the following:

Implicit Casting on Equality Checks

DuckDB performs implicit casting on equality checks, e.g., converting strings to numeric and boolean values. Therefore, there are several instances, where PostgreSQL throws an error while DuckDB successfully computes the result:

Case Sensitivity for Quoted Identifiers

PostgreSQL is case-insensitive. The way PostgreSQL achieves case insensitivity is by lowercasing unquoted identifiers within SQL, whereas quoting preserves case, e.g., the following command creates a table named mytable but tries to query for MyTaBLe because quotes preserve the case.

CREATE TABLE MyTaBLe (x INTEGER);
SELECT * FROM "MyTaBLe";

ERROR:  relation "MyTaBLe" does not exist

PostgreSQL does not only treat quoted identifiers as case-sensitive, PostgreSQL treats all identifiers as case-sensitive, e.g., this also does not work:

CREATE TABLE "PreservedCase" (x INTEGER);
SELECT * FROM PreservedCase;

ERROR:  relation "preservedcase" does not exist

Therefore, case-insensitivity in PostgreSQL only works if you never use quoted identifiers with different cases.

For DuckDB, this behavior was problematic when interfacing with other tools (e.g., Parquet, Pandas) that are case-sensitive by default – since all identifiers would be lowercased all the time. Therefore, DuckDB achieves case insensitivity by making identifiers fully case insensitive throughout the system but preserving their case.

In DuckDB, the scripts above complete successfully:

CREATE TABLE MyTaBLe (x INTEGER);
SELECT * FROM "MyTaBLe";
CREATE TABLE "PreservedCase" (x INTEGER);
SELECT * FROM PreservedCase;
SELECT table_name FROM duckdb_tables();

PostgreSQL's behavior of lowercasing identifiers is accessible using the preserve_identifier_case option:

SET preserve_identifier_case = false;
CREATE TABLE MyTaBLe (x INTEGER);
SELECT table_name FROM duckdb_tables();

However, the case insensitive matching in the system for identifiers cannot be turned off.

Using Double Equality Sign for Comparison

DuckDB supports both = and == for quality comparison, while PostgreSQL only supports =.

SELECT 1 == 1 AS t;

DuckDB returns true, while PostgreSQL returns:

postgres=# SELECT 1 == 1 AS t;
ERROR:  operator does not exist: integer == integer
LINE 1: SELECT 1 == 1 AS t;

Note that the use of == is not encouraged due to its limited portability.

Vacuuming Tables

In PostgreSQL, the VACUUM statement garbage collects tables and analyzes tables. In DuckDB, the VACUUM statement is only used to rebuild statistics. For instruction on reclaiming space, refer to the “Reclaiming space” page.

Strings

Since version 1.3.0, DuckDB escapes characters such as ' in strings serialized in nested data structures. PostgreSQL does not do this.

For an example, run:

SELECT ARRAY[''''];

PostgreSQL returns:

{'}

DuckDB returns:

['\'']

Functions

regexp_extract Function

Unlike PostgreSQL's regexp_substr function, DuckDB's regexp_extract returns empty strings instead of NULLs when there is no match.

to_date Function

DuckDB does not support the to_date PostgreSQL date formatting function. Instead, please use the strptime function.

Resolution of Type Names in the Schema

For CREATE TABLE statements, DuckDB attempts to resolve type names in the schema where a table is created. For example:

CREATE SCHEMA myschema;
CREATE TYPE myschema.mytype AS ENUM ('as', 'df');
CREATE TABLE myschema.mytable (v mytype);

PostgreSQL returns an error on the last statement:

ERROR:  type "mytype" does not exist
LINE 1: CREATE TABLE myschema.mytable (v mytype);

DuckDB runs the statement and creates the table successfully, confirmed by the following query:

DESCRIBE myschema.mytable;

Exploiting Functional Dependencies for GROUP BY

PostgreSQL can exploit functional dependencies, such as i -> j in the following query:

CREATE TABLE tbl (i INTEGER, j INTEGER, PRIMARY KEY (i));
SELECT j
FROM tbl
GROUP BY i;

PostgreSQL runs the query.

DuckDB fails:

Binder Error:
column "j" must appear in the GROUP BY clause or must be part of an aggregate function.
Either add it to the GROUP BY list, or use "ANY_VALUE(j)" if the exact value of "j" is not important.

To work around this, add the other attributes or use the GROUP BY ALL clause.

Behavior of Regular Expression Match Operators

PostgreSQL supports the POSIX regular expression matching operators ~ (case-sensitive partial regex matching) and ~* (case-insensitive partial regex matching) as well as their negated variants, !~ and !~*, respectively.

In DuckDB, ~ is equivalent to regexp_full_match and !~ is equivalent to NOT regexp_full_match. The operators ~* and !~* are not supported.

The table below shows that the correspondence between these functions in PostgreSQL and DuckDB is almost non-existent. We recommend avoiding the POSIX regular expression matching operators in DuckDB.