Mongodb Elemmatch Index |

optimization - MongoDB $elemMatch Index

The query has returned both documents. If we look carefully at the document where _id field equals 1, our requirements are fulfilled in one of the subdocuments of the array, while in the document which _id field equals 2, only one condition is matched in each of the subdocuments of the array. MongoDB is a scalable, high-performance, open source, document-oriented database. It supports a large number of languages and application development platforms.

objects - MongoDB-Indizes für $ elemMatch. mongodb select value in array 1 Basierend auf Ihrer Anfrage stelle ich mir vor, dass Ihre Dokumente in etwa so aussehen. The query works, but it only uses the first key in the compound index, when it should be able to use both. It therefore scans both documents with "ext.s":"s1", instead of just the one matching document. Note that without multi-keys if all inserted documents have a single element in the array, both keys will be used.

In MongoDB the $elemMatch projection operator is used to limits the contents of an array field which is included in the query results to contain only the first. When the find method includes a sort, the find method applies the sort to order the matching documents before it applies the projection. If an array field contains multiple documents with the same field name and the find method includes a sort on that repeating field, the returned documents may not reflect the sort order because the. I'm attempting a descending sort on a subdocument field which is indexed. Without any projection on my query, it appears that my documents use the multikey index's highest value as the sort key. So, as you can see, from 23K documents for this user id, the filter correctly returns 13033 rows, and then the FETCH is executed and also returns 13033, because it is doing the same filter again, so the result will not be different.

  1. This works if an index is built on schedule.time, but not if there is not such index. – Kevin Dolan Feb 1 '11 at 20:13 Kevin: there are some bugs related to indexed/unindexed range queries on arrays which are version dependent.
  2. A collection scan is not being performed. We know that the index is being used because it is given in the "cursor" field. "Index Only" just means that the data returned is completely in the index and the documents themselves don't need to be read. This is not possible with a multi key index as per my previous comment. There is more information.
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在《MongoDB权威指南》这本书中4.3.4 中有个查询文档的章节,里面用到了find和elemMatch,但是给出的例子不是很详细,所以理解起来有点困难,我们来用例子来演示下。0.目的 为了在bl. 博文 来自: 学习、记录、总结、成长. We can see that now there is not need to FETCH documents and it simply performs COUNT_SCAN. I am using MongoDB to store at the moment 100,000 documents. Each document can have from 1 to 150 properties. I need to search by property range for any combination of properties. On average a document has 5 properties set. Here is for example 2 documents, the first has properties 1, 2 and 17 set and doc2 has properties 1, 3 and 45. doc1 =.

03.12.2016 · The $elemMatch query operator. Note that there is also an $elemMatch query operator which performs similar matching, but in the query rather than the results projection. It's not uncommon.11.12.2013 · Tour Start here for a quick overview of the site Help Center Detailed answers to any questions you might have.For MongoDB 3.0 deployments using the WiredTiger storage engine, if you run db.collection.getIndexes from a version of the mongo shell before 3.0 or a version of the driver prior to 3.0 compatible version, db.collection.getIndexes will return no data, even if there are existing indexes.

mongodb - Why is this $elemMatch query not.

10 replies Hi guys, I'm hoping one of you can point out what I'm doing wrong here. I am testing out a collection that will allow users to define custom fields on their documents. I have tucked these custom name/ value pairs into an array called "fields". Here are a couple of example documents to give you a feeling for the structure:"_id. The following query matches only those documents where the results array contains at least one element that is both greater than or equal to 80 and is less than 85. Make sure there are two key/values, one with a high cardinality and one with a very low cardinality - in my example I use simply "high" and "low" to denote these conditions of the value ranges. 由于使用了$ elemMatch组织多个条件查询,mongodb查询的边界为results:[[80,85]。 我们在来看一下(Compound Bounds for Multikey Index) 由于上面的查询条件可以求交集,但是,假如还有一种情.

I think data model is not so good. This is a relations like model. If you can change data model to simple like this: Product: "string", StaticModuleId: "string", Set. In this post, we’ll look at some of the issues MongoDB has with indexes and arrays. Multikey Indexes. In MongoDB, you can index an array field to create an index entry for each element in the array. The resulting index is called a ‘multikey’ index. Multikey indexes can be created over scalar values or embedded documents. 首先说一下 $elemMatch的用法:_id: 1, results: [ 82, 85, 88 ] _id: 2, results: [ 75, 88, 89 ]$elemMatch是匹配document 中数组中至少有一个. The MongoDB $elemMatch operator matches more than one component within an array element.

mongodb - $projection vs $elemMatch - Stack.

基本的Find查询将在其他章节示例,本文主要针对于遇到的问题与解决的方法做个记录,希望可以减少遇到这类问题的同胞-_-! 第一部分 需求与问题 1.1 数据结构 1.1.1. You cannot return multiple elements of an array matching your criteria in any form of a basic.find query. To match more than one element you need to use the.

  1. 21.09.2014 · But index structure think about my imagination above on array can not support this kind of operation because engine can not determine which nodes on index are exactly from a certain array, if only by index. This is the most important point to explain this question.
  2. Mongodb is using the indexes correctly. It all depends on how many docs match your query; Going by the example that I posted, here is how the flow goes: a.If both the fields that are searched have more than one doc id in the index, then the combined index will be used. What that means is.
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