OLAP

Online analitical processing allow users to interactively navigate on the DWH
navigation is done trough chains of operators

OLAP OPERATORS

roll-up

adds aggregation to the data collection

drill-down

removes aggregation from the data collection

slice-and-dice

set a dimension to a specific value reducing the data collection dimensions

pivot

change in layouts of the collection of the data

drill-across

create a liink between data co compare them

drill-through

switches from the multidimensional data model into a operational data

EXTRACTION TRANSFORMATION AND LOADING (ETL)

The ETL process aims to get data from sources, improve general data quality, transform data according to the schema and loads it in the DWH

---
title: ETL
---
flowchart TD
A[EXTRACTION\nextract data from sources]
B[CLEANSING\nimprovements to the quality\nremoving duplicates]
C[TRASFORMATION\ndata processing according to the schema]
D[LOADING\nload data in the DWH]
A --> B
B --> C
C --> D

EXTRACTION

The extraction phase aims to get data from the datasources, there are 2 possible approaches: STATIC or INCREMENTAL

---
title: EXTRACTION
---
flowchart TD
A[APPROACHES]
B[STATIC\nDWH is populated for the first time]
C[INCREMENTAL\nthe DWH is updated with new data regularly]
A --> B & C

Each approach is more suitable for certain types of data:

types of data	types of extraction
structured data (from databases or formatted files )	static (for the first DWH population operation)
unstructured data (from social media)	incremental (for the update operations on the DWH)

CLEANSING

data are processed to improve the quality, data are standardized and mistakes are corrected

SOLUTION FOR DATA INCONSISTENCIES

Dictionary based techniques

they make use of dictionaries and lookup tables to fix typing errors

Aproximate merging

needed when merging data from different sources and there is no common key

TRANSFORMATION

data are altered to match the information schema on the DWH

DENORMALIZATION

for relational database data are rearranged to reduce the number of queries to do on manipulation fase

LOADING

REFRESH

the DWH is completely rewritten with new data

UPDATE

only changes on source are applied to the DWH existent data are not canceled or modified

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