ORDER BY Clause

The ORDER BY clause contains a list of expressions, which can each be attributed with DESC (descending) or ASC (ascending) modifier which determine the sorting direction. If the direction is not specified, ASC is assumed, so it’s usually omitted. The sorting direction applies to a single expression, not to the entire list. Example: ORDER BY Visits DESC, SearchPhrase

Rows that have identical values for the list of sorting expressions are output in an arbitrary order, which can also be non-deterministic (different each time).
If the ORDER BY clause is omitted, the order of the rows is also undefined, and may be non-deterministic as well.

Sorting of Special Values

There are two approaches to NaN and NULL sorting order:

  • By default or with the NULLS LAST modifier: first the values, then NaN, then NULL.
  • With the NULLS FIRST modifier: first NULL, then NaN, then other values.

Example

For the table

  1. ┌─x─┬────y─┐
  2. │ 1 │ ᴺᵁᴸᴸ │
  3. │ 2 │    2 │
  4. │ 1 │  nan │
  5. │ 2 │    2 │
  6. │ 3 │    4 │
  7. │ 5 │    6 │
  8. │ 6 │  nan │
  9. │ 7 │ ᴺᵁᴸᴸ │
  10. │ 6 │    7 │
  11. │ 8 │    9 │
  12. └───┴──────┘

Run the query SELECT * FROM t_null_nan ORDER BY y NULLS FIRST to get:

  1. ┌─x─┬────y─┐
  2. │ 1 │ ᴺᵁᴸᴸ │
  3. │ 7 │ ᴺᵁᴸᴸ │
  4. │ 1 │  nan │
  5. │ 6 │  nan │
  6. │ 2 │    2 │
  7. │ 2 │    2 │
  8. │ 3 │    4 │
  9. │ 5 │    6 │
  10. │ 6 │    7 │
  11. │ 8 │    9 │
  12. └───┴──────┘

When floating point numbers are sorted, NaNs are separate from the other values. Regardless of the sorting order, NaNs come at the end. In other words, for ascending sorting they are placed as if they are larger than all the other numbers, while for descending sorting they are placed as if they are smaller than the rest.

Collation Support

For sorting by String values, you can specify collation (comparison). Example: ORDER BY SearchPhrase COLLATE 'tr' - for sorting by keyword in ascending order, using the Turkish alphabet, case insensitive, assuming that strings are UTF-8 encoded. COLLATE can be specified or not for each expression in ORDER BY independently. If ASC or DESC is specified, COLLATE is specified after it. When using COLLATE, sorting is always case-insensitive.

We only recommend using COLLATE for final sorting of a small number of rows, since sorting with COLLATE is less efficient than normal sorting by bytes.

Implementation Details

Less RAM is used if a small enough LIMIT is specified in addition to ORDER BY. Otherwise, the amount of memory spent is proportional to the volume of data for sorting. For distributed query processing, if GROUP BY is omitted, sorting is partially done on remote servers, and the results are merged on the requestor server. This means that for distributed sorting, the volume of data to sort can be greater than the amount of memory on a single server.

If there is not enough RAM, it is possible to perform sorting in external memory (creating temporary files on a disk). Use the setting max_bytes_before_external_sort for this purpose. If it is set to 0 (the default), external sorting is disabled. If it is enabled, when the volume of data to sort reaches the specified number of bytes, the collected data is sorted and dumped into a temporary file. After all data is read, all the sorted files are merged and the results are output. Files are written to the /var/lib/clickhouse/tmp/ directory in the config (by default, but you can use the tmp_path parameter to change this setting).

Running a query may use more memory than max_bytes_before_external_sort. For this reason, this setting must have a value significantly smaller than max_memory_usage. As an example, if your server has 128 GB of RAM and you need to run a single query, set max_memory_usage to 100 GB, and max_bytes_before_external_sort to 80 GB.

External sorting works much less effectively than sorting in RAM.

Optimization of Data Reading

If ORDER BY expression has a prefix that coincides with the table sorting key, you can optimize the query by using the optimize_read_in_order setting.

When the optimize_read_in_order setting is enabled, the Clickhouse server uses the table index and reads the data in order of the ORDER BY key. This allows to avoid reading all data in case of specified LIMIT. So queries on big data with small limit are processed faster.

Optimization works with both ASC and DESC and doesn’t work together with GROUP BY clause and FINAL modifier.

When the optimize_read_in_order setting is disabled, the Clickhouse server does not use the table index while processing SELECT queries.

Consider disabling optimize_read_in_order manually, when running queries that have ORDER BY clause, large LIMIT and WHERE condition that requires to read huge amount of records before queried data is found.

Optimization is supported in the following table engines:

In MaterializedView-engine tables the optimization works with views like SELECT ... FROM merge_tree_table ORDER BY pk. But it is not supported in the queries like SELECT ... FROM view ORDER BY pk if the view query doesn’t have the ORDER BY clause.

ORDER BY Expr WITH FILL Modifier

This modifier also can be combined with LIMIT … WITH TIES modifier.

WITH FILL modifier can be set after ORDER BY expr with optional FROM expr, TO expr and STEP expr parameters.
All missed values of expr column will be filled sequentially and other columns will be filled as defaults.

Use following syntax for filling multiple columns add WITH FILL modifier with optional parameters after each field name in ORDER BY section.

  1. ORDER BY expr [WITH FILL] [FROM const_expr] [TO const_expr] [STEP const_numeric_expr], ... exprN [WITH FILL] [FROM expr] [TO expr] [STEP numeric_expr]

WITH FILL can be applied only for fields with Numeric (all kind of float, decimal, int) or Date/DateTime types.
When FROM const_expr not defined sequence of filling use minimal expr field value from ORDER BY.
When TO const_expr not defined sequence of filling use maximum expr field value from ORDER BY.
When STEP const_numeric_expr defined then const_numeric_expr interprets as is for numeric types as days for Date type and as seconds for DateTime type.
When STEP const_numeric_expr omitted then sequence of filling use 1.0 for numeric type, 1 day for Date type and 1 second for DateTime type.

For example, the following query

  1. SELECT n, source FROM (
  2.    SELECT toFloat32(number % 10) AS n, 'original' AS source
  3.    FROM numbers(10) WHERE number % 3 = 1
  4. ) ORDER BY n

returns

  1. ┌─n─┬─source───┐
  2. │ 1 │ original │
  3. │ 4 │ original │
  4. │ 7 │ original │
  5. └───┴──────────┘

but after apply WITH FILL modifier

  1. SELECT n, source FROM (
  2.    SELECT toFloat32(number % 10) AS n, 'original' AS source
  3.    FROM numbers(10) WHERE number % 3 = 1
  4. ) ORDER BY n WITH FILL FROM 0 TO 5.51 STEP 0.5

returns

  1. ┌───n─┬─source───┐
  2. │   0 │          │
  3. │ 0.5 │          │
  4. │   1 │ original │
  5. │ 1.5 │          │
  6. │   2 │          │
  7. │ 2.5 │          │
  8. │   3 │          │
  9. │ 3.5 │          │
  10. │   4 │ original │
  11. │ 4.5 │          │
  12. │   5 │          │
  13. │ 5.5 │          │
  14. │   7 │ original │
  15. └─────┴──────────┘

For the case when we have multiple fields ORDER BY field2 WITH FILL, field1 WITH FILL order of filling will follow the order of fields in ORDER BY clause.

Example:

  1. SELECT
  2.     toDate((number * 10) * 86400) AS d1,
  3.     toDate(number * 86400) AS d2,
  4.     'original' AS source
  5. FROM numbers(10)
  6. WHERE (number % 3) = 1
  7. ORDER BY
  8.     d2 WITH FILL,
  9.     d1 WITH FILL STEP 5;

returns

  1. ┌───d1───────┬───d2───────┬─source───┐
  2. │ 1970-01-11 │ 1970-01-02 │ original │
  3. │ 1970-01-01 │ 1970-01-03 │          │
  4. │ 1970-01-01 │ 1970-01-04 │          │
  5. │ 1970-02-10 │ 1970-01-05 │ original │
  6. │ 1970-01-01 │ 1970-01-06 │          │
  7. │ 1970-01-01 │ 1970-01-07 │          │
  8. │ 1970-03-12 │ 1970-01-08 │ original │
  9. └────────────┴────────────┴──────────┘

Field d1 doesn’t fill and use default value cause we don’t have repeated values for d2 value, and sequence for d1 can’t be properly calculated.

The following query with a changed field in ORDER BY

  1. SELECT
  2.     toDate((number * 10) * 86400) AS d1,
  3.     toDate(number * 86400) AS d2,
  4.     'original' AS source
  5. FROM numbers(10)
  6. WHERE (number % 3) = 1
  7. ORDER BY
  8.     d1 WITH FILL STEP 5,
  9.     d2 WITH FILL;

returns

  1. ┌───d1───────┬───d2───────┬─source───┐
  2. │ 1970-01-11 │ 1970-01-02 │ original │
  3. │ 1970-01-16 │ 1970-01-01 │          │
  4. │ 1970-01-21 │ 1970-01-01 │          │
  5. │ 1970-01-26 │ 1970-01-01 │          │
  6. │ 1970-01-31 │ 1970-01-01 │          │
  7. │ 1970-02-05 │ 1970-01-01 │          │
  8. │ 1970-02-10 │ 1970-01-05 │ original │
  9. │ 1970-02-15 │ 1970-01-01 │          │
  10. │ 1970-02-20 │ 1970-01-01 │          │
  11. │ 1970-02-25 │ 1970-01-01 │          │
  12. │ 1970-03-02 │ 1970-01-01 │          │
  13. │ 1970-03-07 │ 1970-01-01 │          │
  14. │ 1970-03-12 │ 1970-01-08 │ original │
  15. └────────────┴────────────┴──────────┘