Aliasing

When we join two data streams with the sum operator, a new data schema appears. We can refer to the successive values of this schema through the name of the data stream, indexed sequentially from the start of the schema.

We can, however, also use the names the stream was built from. A value can be pointed to both by the output stream’s name, indexed from the start of the schema, and by the source stream’s name, shifted relative to its join position.

The example uses the canonical declarations used throughout the chapter:

DECLARE a BYTE, b INTEGER \
STREAM core0, 0.1 \
FILE 'sensor_a.txt'

DECLARE c INTEGER, d FLOAT \
STREAM core1, 0.2 \
FILE 'sensor_b.txt'

SELECT merged[0], merged[2], core0[0], core1[0] \
STREAM merged \
FROM core0 + core1

After compilation we get:

$ xretractor -c query.rql
merged(1/10)
        :- PUSH_STREAM(core0)
        :- PUSH_STREAM(core1)
        :- STREAM_ADD
        merged_0: BYTE
                PUSH_ID(merged[0])
        merged_1: INTEGER
                PUSH_ID(merged[2])
        merged_2: BYTE
                PUSH_ID(merged[0])
        merged_3: INTEGER
                PUSH_ID(merged[2])
core0(1/10)     sensor_a.txt
        a: BYTE
        b: INTEGER
core1(1/5)      sensor_b.txt
        c: INTEGER
        d: FLOAT

merged[0] and core0[0] both end up as PUSH_ID(merged[0]) — they are the same field. But core1[0] — the first field of core1’s schema — ends up as PUSH_ID(merged[2]), not merged[0]. The compiler translated the local index core1[0] into an absolute position in the combined schema: core0 occupies positions 0 and 1, so core1 starts at position 2. What if we replace the + operator with #? I encourage you to experiment.

NOTE: The functionality described here is covered by the test: Pattern7, described in the appendix Integration Tests.