Asterisk Expansion

Everyone who has written SQL knows the magic * character in that language. Invoking a SELECT command with this argument expands the argument list based on the schemas of the tables produced by relational joins. I wanted to achieve something similar in RQL.

The example uses the canonical declarations used throughout the chapter:

DECLARE a BYTE, b INTEGER STREAM core0, 0.1 TEXTFILE 'sensor_a.txt'
DECLARE c INTEGER, d FLOAT STREAM core1, 0.2 TEXTFILE 'sensor_b.txt'

SELECT *         STREAM merged FROM core0 + core1
SELECT merged[2] STREAM result FROM merged

Let’s compile it and see the effect:

core0(1/10)	sensor_a.txt
	a: BYTE
	b: INTEGER
core1(1/5)	sensor_b.txt
	c: INTEGER
	d: FLOAT
merged(1/10)	tail=1
	:- PUSH_STREAM(core0)
	:- PUSH_STREAM(core1)
	:- STREAM_ADD
	core0_0: BYTE
		PUSH_ID(merged[0])
	core0_1: INTEGER
		PUSH_ID(merged[1])
	core1_2: INTEGER
		PUSH_ID(merged[2])
	core1_3: FLOAT
		PUSH_ID(merged[3])
result(1/10)	tail=1
	:- PUSH_STREAM(merged)
	result_0: INTEGER
		PUSH_ID(result[2])

The * symbol turned into four fields: core0_0, core0_1, core1_2, core1_3. Naming convention: source stream name + absolute position in the output schema. Field types determine the order - core0 contributes BYTE and INTEGER at positions 0 and 1, core1 contributes INTEGER and FLOAT at positions 2 and 3. Referring to merged[2] in the result query gets us a field of type INTEGER - third in order, the first field from core1.

NOTE: The functionality described here is covered by the test: Pattern3, described in the appendix Integration Tests.