3,523 research outputs found
Evaluation of optimization techniques for aggregation
Aggregations are almost always done at the top of operator tree after all selections
and joins in a SQL query. But actually they can be done before joins and make later
joins much cheaper when used properly. Although some enumeration algorithms
considering eager aggregation are proposed, no sufficient evaluations are available
to guide the adoption of this technique in practice. And no evaluations are done
for real data sets and real queries with estimated cardinalities. That means it is not
known how eager aggregation performs in the real world.
In this thesis, a new estimation method for group by and join combining traditional
estimation method and index-based join sampling is proposed and evaluated.
Two enumeration algorithms considering eager aggregation are implemented and
compared in the context of estimated cardinality. We find that the new estimation
method works well with little overhead and that under certain conditions, eager
aggregation can dramatically accelerate queries
MISO Networks with Imperfect CSIT: A Topological Rate-Splitting Approach
Recently, the Degrees-of-Freedom (DoF) region of multiple-input-single-output
(MISO) networks with imperfect channel state information at the transmitter
(CSIT) has attracted significant attentions. An achievable scheme is known as
rate-splitting (RS) that integrates common-message-multicasting and
private-message-unicasting. In this paper, focusing on the general -cell
MISO IC where the CSIT of each interference link has an arbitrary quality of
imperfectness, we firstly identify the DoF region achieved by RS. Secondly, we
introduce a novel scheme, so called Topological RS (TRS), whose novelties
compared to RS lie in a multi-layer structure and transmitting multiple common
messages to be decoded by groups of users rather than all users. The design of
TRS is motivated by a novel interpretation of the -cell IC with imperfect
CSIT as a weighted-sum of a series of partially connected networks. We show
that the DoF region achieved by TRS covers that achieved by RS. Also, we find
the maximal sum DoF achieved by TRS via hypergraph fractional packing, which
yields the best sum DoF so far. Lastly, for a realistic scenario where each
user is connected to three dominant transmitters, we identify the sufficient
condition where TRS strictly outperforms conventional schemes.Comment: submitted for publicatio
Achievable Sum DoF of the K-User MIMO Interference Channel with Delayed CSIT
This paper considers a -user multiple-input-multiple-output (MIMO)
interference channel (IC) where 1) the channel state information obtained by
the transmitters (CSIT) is completely outdated, and 2) the number of transmit
antennas at each transmitter, i.e., , is greater than the number of receive
antennas at each user, i.e., . The usefulness of the delayed CSIT was
firstly identified in a -phase Retrospective Interference Alignment (RIA)
scheme proposed by Maddah-Ali et al for the Multiple-Input-Single-Output
Broadcast Channel, but the extension to the MIMO IC is a non-trivial step as
each transmitter only has the message intended for the corresponding user.
Recently, Abdoli et al focused on a Single-Input-Single-Output IC and solved
such bottleneck by inventing a -phase RIA with distributed overheard
interference retransmission. In this paper, we propose two -phase RIA
schemes suitable for the MIMO IC by generalizing and integrating some key
features of both Abdoli's and Maddah-Ali's works. The two schemes jointly yield
the best known sum Degrees-of-Freedom (DoF) performance so far. For the case
, the achieved sum DoF is asymptotically given by
when
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