Top-k queries on RDF graphs

Wang, Dong; Zou, Lei; Zhao, Dongyan

doi:10.1016/j.ins.2015.04.032

Cited by 15 publications

(5 citation statements)

References 29 publications

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“…Fischer et al (2009) found from a statistical analysis that magnetic elements formed via convective collapse are mostly destroyed by flux cancellation. Cancellation is also observed to play a significant role in many of the upper atmospheric dynamic phenomena such as flares, filament eruptions and coronal mass ejections (e.g., Wang et al, 1996;Zhang et al, 2001;Yardley et al, 2016), X-ray bright points (Priest et al, 1994;Harvey, 1996;Zhao et al, 2017), Ellerman bombs (Georgoulis et al, 2002;Watanabe et al, 2008;Vissers et al, 2015;Reid et al, 2016), the formation of prominences (Martin, 1988), coronal jets (Panesar et al, 2017), etc.…”

Section: Introductionmentioning

confidence: 99%

Cancelation of small-scale magnetic features

Kaithakkal

Solanki

2019

A&A

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Aims. We investigate small-scale flux cancellations in a young active region observed with the high-resolution imaging magnetograph IMaX on the Sunrise balloon-borne solar observatory. Methods. The observed Stokes profiles of the photospheric Fe i 5250.2 Å line are inverted using the SPINOR code to obtain the atmospheric parameters, including magnetic field vector and the line-of-sight velocity. We then identified 11 opposite-polarity cancelling pairs using an automatic detection code, studied their evolution in detail, and derived their statistical properties. We classified the cancellations into two groups. Class I events are those for which cancellation happens between a pre-existing large magnetic feature of one polarity and a smaller feature of the other polarity that emerged/appeared nearby. For Class II events cancellations occur between two pre-existing, previously unconnected features that converge toward each other. Results. All studied events have an apparent cancellation time less than 10 minutes and display a significant transient linear polarization signal along the polarity inversion line. The cancellation events are characterized by a flux decay rate of about 10 15 Mx s −1 . For Class I events, the Doppler velocity of the disappearing patch gradually switches from blueshift during the initial phase of cancellation to redshift towards the end of the cancellation. For class II events, the Doppler velocity is consistently redshifted. Horizontal convergence speeds of Class II pairs fall between 0.3 and 1.22 km s −1 . The elements often do not converge directly towards each other, so that the proper motion speeds of the individual elements is higher, in the range of 1 -2.7 km s −1 .Conclusions. We propose that these cancellation events result from either field-line submergence (Class I), or reconnection followed by submergence (Class II and/or Class I). Ohmic dissipation of magnetic energy could also play a role for both classes. The dynamics and evolution of these events are influenced by neighbouring granular motions. We propose that, at least for the Class II events, the granular motions could possibly be driving magnetic reconnection, rather than the supergranular motions proposed for the larger cancellation events studied earlier. Specific flux cancellation rates of the Class II events seem to indicate that they belong to somewhat different category of cancellations when compared with those studied in SOT/Hinode and MDI/SOHO data.

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Section: Introductionmentioning

confidence: 99%

Cancelation of small-scale magnetic features

Kaithakkal

Solanki

2019

A&A

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“…In case of (56), we may proceed by cases considering the condition (36). In a more detail, let D 1 be an RDT on R ∪ S such that R ∩ S = ∅, D 2 be an RDT on S, and D 3 be an RDT on R. Furthermore, assume that for a given r ∈ Tupl(R) and all s ∈ Tupl(S), we have that D 2 (s) > D 1 (rs) implies D 3 (r) ≤ D 1 (rs).…”

Section: Invariance Theoremsmentioning

confidence: 99%

“…It remains to prove the equality in the case when (36) does not hold. That is, assume that for given r ∈ Tupl(R) there is s ∈ Tupl(S) such that D 2 (s) > D 1 (rs) and D 3 (r) > D 1 (rs).…”

Section: Invariance Theoremsmentioning

confidence: 99%

“…Therefore, Consequence 1 describes an important property of a particular query system supporting top-k queries. Systems supporting top-k queries are recently gaining interest [29,32,36] and investigations in this direction may be exploited as optimization techniques. For instance, in case of reiterated queries executed with different parameters, observations like Consequence 1 may help identify that certain changes in input parameters will have no influence on the results of top-k queries.…”

Section: Introductionmentioning

confidence: 99%

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Invariance to ordinal transformations in rank-aware databases

Vychodil

2017

Information Sciences

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We study influence of ordinal transformations on results of queries in rank-aware databases which derive their operations with ranked relations from totally ordered structures of scores with infima acting as aggregation functions. We introduce notions of ordinal containment and equivalence of ranked relations and prove that infima-based algebraic operations with ranked relations are invariant to ordinal transformations: Queries applied to original and transformed data yield results which are equivalent in terms of the order given by scores, meaning that top-k results of queries remain the same. We show this important property is preserved in alternative query systems based of relational calculi developed in context of G\"odel logic. We comment on relationship to monotone query evaluation and show that the results can be attained in alternative rank-aware approaches

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“…Thus, when the right-side index is large, the performance of rank join suffers. In another framework introduced by Wang et al [36], quantitative entities in the RDF dataset are separated out into an MS-tree index. In the first step of query processing, candidate entities are located using the MS-tree index that are then used as seeds for performing breadth-first (BFS) traversals over the graph to find matching sub-graphs.…”

Section: Top-k Queries On Graphsmentioning

confidence: 99%

Spec-QP: Speculative Query Planning for Joins over Knowledge Graphs

Mohanty,

Ramanath,

Yahya

et al. 2017

Preprint

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Organisations store huge amounts of data from multiple heterogeneous sources in the form of Knowledge Graphs (KGs). One of the ways to query these KGs is to use SPARQL queries over a database engine. Since SPARQL follows exact match semantics, the queries may return too few or no results. Recent works have proposed query relaxation where the query engine judiciously replaces a query predicate with similar predicates using weighted relaxation rules mined from the KG. The space of possible relaxations is potentially too large to fully explore and users are typically interested in only top-k results, so such query engines use top-k algorithms for query processing. However, they may still process all the relaxations, many of whose answers do not contribute towards top-k answers. This leads to computation overheads and delayed response times.We propose Spec-QP, a query planning framework that speculatively determines which relaxations will have their results in the top-k answers. Only these relaxations are processed using the top-k operators. We, therefore, reduce the computation overheads and achieve faster response times without adversely affecting the quality of results. We tested Spec-QP over two datasets -XKG and Twitter, to demonstrate the efficiency of our planning framework at reducing runtimes with reasonable accuracy for query engines supporting relaxations.

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Top-k queries on RDF graphs

Cited by 15 publications

References 29 publications

Cancelation of small-scale magnetic features

Cancelation of small-scale magnetic features

Invariance to ordinal transformations in rank-aware databases

Spec-QP: Speculative Query Planning for Joins over Knowledge Graphs

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