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For many operations, DuckDB preserves the order of rows, similarly to data frame libraries such as Pandas.
Example
Take the following table for example:
CREATE TABLE tbl AS
SELECT *
FROM (VALUES (1, 'a'), (2, 'b'), (3, 'c')) t(x, y);
SELECT *
FROM tbl;
x | y |
---|---|
1 | a |
2 | b |
3 | c |
Let's take the following query that returns the rows where x
is an odd number:
SELECT *
FROM tbl
WHERE x % 2 == 1;
x | y |
---|---|
1 | a |
3 | c |
Because the row (1, 'a')
occurs before (3, 'c')
in the original table, it is guaranteed to come before that row in this table too.
Clauses
The following clauses guarantee that the original row order is preserved:
COPY
(see Insertion Order)FROM
with a single tableLIMIT
OFFSET
SELECT
UNION ALL
WHERE
- Window functions with an empty
OVER
clause
Tip
row_number() OVER ()
allows turning the original row order into an explicit column that can be referenced in the operations that don't preserve row order by default. On materialized tables, therowid
pseudo-column can be used to the same effect.
The following operations do not guarantee that the row order is preserved:
FROM
with multiple tables and/or subqueriesJOIN
UNION
USING SAMPLE
GROUP BY
(in particular, the output order is undefined and the order in which rows are fed into order-sensitive aggregate functions is undefined unless explicitly specified in the aggregate function)ORDER BY
(specifically,ORDER BY
may not use a stable algorithm)
Insertion Order
By default, the following components preserve insertion order:
- CSV reader (
read_csv
function) - JSON reader (
read_json
function) - Parquet reader (
read_parquet
function)
Preservation of insertion order is controlled by the preserve_insertion_order
configuration option.
This setting is true
by default, indicating that the order should be preserved.
To change this setting, use:
SET preserve_insertion_order = false;