The ORDER BY clause allows you to specify the columns, or expressions, by which a relational (non-streaming) query is sorted. If you do not specify an ORDER BY clause, the order of the rows is unspecified.
ORDER BY is also supported for streaming queries (see Streaming Order By below) and, by using a SQLstream keyword, can be used to time-sort rows in a time-based window. See T-sorting Stream Input below for more details.
This topic contains information on teh following:
Note: For the use and effect of the t_sort_clause, see T-sorting Stream Input below.
The ORDER BY clause is evaluated after FROM, WHERE, GROUP BY, HAVING, and SELECT clauses. As a result, expressions that are filtered away by a GROUP BY or SELECT DISTINCT operation cannot be sorted on.
For example, it is illegal to write
SELECT DISTINCT deptno FROM emp ORDER BY gender
because for a given value of deptno there is no single well-defined value of gender.
If an expression has been given a column alias in the SELECT clause, you can refer to it by that alias.
If the expression is an integer literal n, it means the nth entry in the SELECT clause.
For example, the following three queries are equivalent:
SELECT orderId, unitPrice * quantity AS total FROM Orders ORDER BY 2; SELECT orderId, unitPrice * quantity AS total FROM Orders ORDER BY total; SELECT orderId, unitPrice * quantity AS total FROM Orders ORDER BY unitPrice * quantity;
A streaming query can use ORDER BY if its leading expression is time-based and monotonic. For example, a streaming query whose leading expression is based on the ROWTIME column can use ORDER BY to do the following operations:
The “time-based and monotonic” requirement on the leading expression means that the query
SELECT STREAM DISTINCT ticker FROM trades ORDER BY ticker;
will fail, but the query
SELECT STREAM DISTINCT rowtime, ticker FROM trades ORDER BY ROWTIME, ticker;
Streaming ORDER BY sorts rows using SQL standard compliant syntax for the ORDER BY clause. It can be combined with a UNION ALL statement, and can sort on expressions, such as:
SELECT STREAM x, y FROM t1 UNION ALL SELECT STREAM a, b FROM t2 ORDER BY ROWTIME, MOD(x, 5)
The ORDER BY clause can specify ascending or descending sort order, and can use column ordinals, as well as ordinals specifying (referring to) the position of items in the select list.
The streaming ORDER BY clause includes the following functional attributes:
Streaming ORDER BY can include the pseudo-column ROWTIME. As a pseudo-column, ROWTIME behaves as a stream column, but is not stored in the stream.
Note: Unadorned ROWTIME (not qualified with a stream name) can only be specified in the SELECT list or the ORDER BY clause.
The following query is an example of streaming ORDER BY:
SELECT STREAM state, city, SUM(amount) FROM orders GROUP BY FLOOR(ROWTIME TO HOUR), state, city ORDER BY FLOOR(ROWTIME TO HOUR), state, SUM(amount);
SQLstream s-Server’s real-time analytics use the fact that arriving data is ordered by ROWTIME. However, sometimes data arriving from multiple sources may not be time-synchronized.
While SQLstream s-Server can sort data from individual data sources that have been independently inserted into s-Server’s native stream, in some cases such data may have already combined from multiple sources (such as for efficient consumption at an earlier stage in processing). At other times, high volume data sources could make direct insertion impossible.
In addition, an unreliable data source could block progress by forcing s-Server to wait indefinitely, unable to proceed until all connected data sources deliver. In this case, data from this source could be unsynchronized.
SQLstream s-Server resolves these issues using an execution object (XO), the t-Sort XO. It uses a sliding time-based window of incoming rows to reorder those rows by ROWTIME.
You specify the time-based parameter for sorting and the time-based window in which the streaming rows are to be time-sorted, using the following syntax:
ORDER BY <timestamp_expr> WITHIN <interval_literal>
The SQL statements below pre-sort rows from different streams using windows of two minutes and two seconds.
CREATE OR REPLACE STREAM "vehicle_positions" ( "PositionTime" TIMESTAMP, "VID" INTEGER, "vSpeed" SMALLINT, "vBearing" REAL) DESCRIPTION 'TSort Example'; SELECT STREAM "PositionTime" AS ROWTIME, VID, "vSpeed", "vBearing" FROM "vehicle_positions" ORDER BY "PositionTime" WITHIN INTERVAL '2' MINUTE, VID; SET SCHEMA '"SALES"'; CREATE OR REPLACE STREAM "ASKS" ( "TransactionTime" TIMESTAMP, "time" BIGINT, "ticker" VARCHAR(5), "shares" INTEGER, "price" REAL, "expiryMs" BIGINT, "comment" VARCHAR(1024)) DESCRIPTION 'updating ASKS for this example'; SELECT STREAM "TransactionTime" AS ROWTIME, "ticker", "price" FROM ASKS ORDER BY "TransactionTime" WITHIN INTERVAL '2' SECOND;
The T-sort XO has the following restrictions:
If any of these requirements are not met, the statement will fail with errors. Additional notes:
It is also possible to combine a T-Sort with a GROUP BY, either explicitly, or implicitly as a result of a planner optimization.
(This SQL feature has been available since version 7.2.4).
The T-Sort + GROUP BY can be explicitly stated in the GROUP BY clause - see Grouping by non-monotonic timestamp column
(This optimization has been in effect since version 7.0.4).
The two operations can be implicitly combined as a result of the SQLstream optimizer opportunistically combining a T-Sort operation with an immediately following GROUP BY operation.
If the planner recognizes that a SELECT STREAM … ORDER BY .. WITHIN view ia immediately followed by a SELECT STREAM … GROUP BY … it generates a plan that combines these.
This will only happen if the two XOs are in the same streaming graph. Normally that means they are both executed as part of the same pump.
These benefits apply equally to both explicit and implicit combination of T-Sort and GROUP BY.
Rather than a T-Sort XO saving every row into a queue, and then sending it to a second Streaming Aggregation XO performing the aggregation, the Streaming Aggregation XO handles both:
If we compare the memory required:
There is also a processing benefit: