Hi all,
I am using Virtuoso build having SHA =
b86c2a70106138325ed53c60690d7c8f79bc5446 and I found a weird behaviour
in the queries making use of literals. Basically, the problem is that
the query plan changes with no apparent reason depending on whether the
same query is run immediately after starting the server or some time
(and queries) later, and such change causes a difference in the
execution time (the first time the query is faster).
The query has the following pattern:
SELECT *
WHERE {
?id <http://example.com#relation> "Some text" .
?id <http://example.com#relation> "Some other text" .
}
You can find the two different query plans in the bottom, the former
being the "fast" one and the latter being the "slow" one. What's even
more strange is that, if the literals are moved in a FILTER clause and
"cast" using the STR() function, the query is always executed in "fast
mode". In other words, if the query is written in this way, the plan
does not change (last query plan in the bottom):
SELECT *
WHERE {
?id <http://example.com#relation> "Some text" .
?id <http://example.com#relation> ?o . FILTER(STR(?o) = "Some other text")
}
It looks like the main difference, besides the FILTER addition (the
"After test" block), is the presence of the "inlined" keyword and the
use of the RDF_QUAD index rather than the POGS index. Can you explain
what's happening with the indices and what does the "inlined" keyword mean?
Thank you,
Nicola
-------------------------------------
First ("fast") query plan:
{
time 0.038% fanout 1 input 1 rows
time 1% fanout 902 input 1 rows
Precode:
0: __rdflit := Call __rdflit (rdflit6325)
5: __rdflit := Call __rdflit (rdflit1206)
10: BReturn 0
RDF_QUAD_POGS 9e+02 rows(s_9_4_t0.S)
P = ##relation , O = rdflit6325
time 99% fanout 0.00776053 input 902 rows
RDF_QUAD 0.01 rows()
inlined P = ##relation , S = s_9_4_t0.S , O = rdflit1206
After code:
0: id := Call __id2in (s_9_4_t0.S)
5: BReturn 0
time 0.016% fanout 0 input 7 rows
Select (id)
}
8 msec 40% cpu, 917 rnd 1810 seq 0% same seg
80.8279% same pg
513 disk reads, 508 read ahead, 29.745% wait
Compilation: 13 msec 702 reads 25.6666% read 0 messages 0% clw
------------
Second ("slow") query plan:
{
time 4.6e-05% fanout 1 input 1 rows
time 0.001% fanout 902 input 1 rows
Precode:
0: __rdflit := Call __rdflit (rdflit6325)
5: __rdflit := Call __rdflit (rdflit1206)
10: BReturn 0
RDF_QUAD_POGS 0.065 rows(s_9_4_t0.S)
P = ##relation , O = rdflit6325
time 1e+02% fanout 0.00776053 input 902 rows
RDF_QUAD_POGS 4.6e-14 rows()
P = ##relation , O = rdflit1206 S = s_9_4_t0.S
After code:
0: id := Call __id2in (s_9_4_t0.S)
5: BReturn 0
time 2e-05% fanout 0 input 7 rows
Select (id)
}
5995 msec 99% cpu, 903 rnd 1.37377e+08 seq 0% same seg
0% same pg
Compilation: 1 msec 0 reads 0% read 0 messages 0% clw
-------------
Plan with FILTER and STR():
{
time 0.12% fanout 1 input 1 rows
time 3.1% fanout 902 input 1 rows
Precode:
0: __rdflit := Call __rdflit (rdflit6325)
5: BReturn 0
RDF_QUAD_POGS 0.065 rows(s_5_4_t0.S)
P = ##relation , O = rdflit6325
time 47% fanout 1.01663 input 902 rows
RDF_QUAD 0.46 rows(s_5_4_t1.O)
inlined P = ##relation , S = s_5_4_t0.S
time 50% fanout 0.00763359 input 917 rows
END Node
After test:
0: __rdf_strsqlval := Call __rdf_strsqlval (s_5_4_t1.O, 0 )
5: if (<c Some other text> = __rdf_strsqlval) then 9 else 10 unkn 10
9: BReturn 1
10: BReturn 0
After code:
0: id := Call __id2in (s_5_4_t0.S)
5: o := Call __ro2sq (s_5_4_t1.O)
10: BReturn 0
time 0.0084% fanout 0 input 7 rows
Select (id, o)
}
2 msec 67% cpu, 1827 rnd 2461 seq 0% same seg
40.5908% same pg
Compilation: 1 msec 0 reads 0% read 0 messages 0% clw
------------------------------------------------------------------------------
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