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How a regular expression is used for searching documents in mongoDB ?

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How a regular expression is used for searching documents in mongoDB ?
posted Jun 14, 2015 by Vikram Singh

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Does anyone know how I remove orphaned documents in a cluster (sharding) in mongodb version 3.0.10, My actual data are 20 million orphans and documents the value amounts to 21 million data

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Here I would like to explain my query a bit more.

I inserted multiple documents within a collection. Few documents have same set of keys and few of them have some extra key:value pairs . For example:
document 1 : key1 : "key1"
document 2: key1 : "key1", key2: "key2"
document 3: key1 : "key1", key2: "key2", key3 : "key3"
document 4: key1 : "key1", key3 : "key3"

Now I want to delete the documents which have key1 and key2.
What will be the command to do so ?

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I have a project, which has 1000 images. From each image, I get 1 million documents that I currently store in a mongoDB collection.

I have 2 alternatives,
1. Create a separate collection for each image, over multiple projects, say 10 projects, and each image(collection) would have 1 million documents that will be indexed. OR
2. Have a single collection and have 1000x1,000,000 documents in one collection, which again will be indexed.

My research and reading suggests that it is not a very good idea to create 1000 collections (for each image) and create an index for each one of 1000, and go with adding a billion documents to a single collection, to which mongoDB caters with no major issue.
My main aim is to enhance performance for reading. There would be no updates and deletes. Only reads and writes. I can afford writes to be slow, but not reads. So am I right in choosing the 2nd alternative?

Any guidance is highly appreciated.

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