NoSQL for Mere Mortals by Dan Sullivan

Author:Dan Sullivan [Sullivan, Dan]
Language: eng
Format: epub, azw3, pdf
Publisher: Addison-Wesley Professional
Published: 2015-04-23T21:00:00+00:00

Query Processor

Getting data from a document database is more complicated than getting it from key-value databases. Remember, if you have a key, you can retrieve an associated value from a key-value database.

Document databases offer more options for retrieving data. For example, you could retrieve documents created before a particular date, or documents that are a specific type, documents that contain the string “long distance running” in a product description, or some combination of all of these.

The query processor is an important part of a database management system. It takes as input queries and data about the documents and collections in the database and produces a sequence of operations that retrieve the selected data.

Key-value databases do not need query processors; they function by looking up values by keys. There is no need to analyze logical statements such as the following:

Click here to view code image

(createDate > '1-Jan-2015') AND (productType =

'electronics')

When there can be multiple conditions on selecting documents, the query processor must make decisions, such as which criteria it should apply first. Should it find all documents with a creation date greater than January 1, 2015, or should it retrieve all documents about electronics products?

If there are fewer documents with a creation date after January 1, 2015, than there are documents with an electronics type, then it would make sense to retrieve documents based on creation date because it will return fewer documents than the other criteria. This means the second criterion is applied to a smaller number of documents.

This is a simple example of the kinds of options a query processor evaluates as it builds its plan to retrieve data.

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