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https://bugs.launchpad.net/bugs/1214538

Title:
  Not indexing documents unless all fields are in the index expression
  clause

Status in U1DB Qt/ QML:
  Fix Committed
Status in “u1db-qt” package in Ubuntu:
  Fix Released

Bug description:
  It seems that queries only return results of documents, where all the
  document's fields are indexed [regardless of whether or not those
  fields are being queried].

  I've included a number of examples below, I hope they clarify the
  situation:

  Examples:

  Database definition:

      U1db.Database {
          id: theDB
          path: "indextest.db"
      }

  Two sample documents: the first has only a "title" field, the second
  has both "title" and "num" fields:

      U1db.Document {
          database: theDB
          create: true
          docId: "vic"
          defaults: {"title": "vic"}
      }

      U1db.Document {
          database: theDB
          create: true
          docId: "xqwzts"
          defaults: {
              "title": "xqwzts",
              "num": "123"
          }
      }

  Index only the title field since it is what I care about querying:

      U1db.Index {
          database: theDB
          id: theIndex
          expression: ["title"]
      }

  An empty query which should match all documents in the index:

      U1db.Query {
          id: theQuery
          index: theIndex
      }

  Expected result: both documents should be returned

  Action result: only the first document is returned

  Detailed output:

  JSON.stringify(theDB.listDocs())

      {"0":"vic","1":"xqwzts"}

  JSON.stringify(theIndex)

  {"database":{"error":"","objectName":"","path":"indextest.db"},"name":"","objectName":"","expression":{"0":"title"}}

  JSON.stringify(theQuery)

  {"index":{"database":{"error":"","objectName":"","path":"indextest.db"},"name":"","objectName":"","expression":{"0":"title"}},"objectName":"","results":[{"title":"vic"}],"documents":{"0":"vic"}}

  Specifying the query to search for title = "*" makes no difference:

      U1db.Query {
          id: theQuery
          index: theIndex
          query: [{"title": "*"}]
      }

  JSON.stringify(theQuery)

  {"index":{"database":{"error":"","objectName":"","path":"indextest.db"},"name":"","objectName":"","expression":{"0":"title"}},"objectName":"","results":[{"title":"vic"}],"documents":{"0":"vic"},"query":[{"title":"*"}]}

  It seems the second document simply isn't in the index, so no query
  will return it.

  To add the second document to the index we must add all fields of the
  document to the index expression:

      U1db.Index {
          database: theDB
          id: theIndex
          expression: ["title", "num"]
      }

  And remove the query clause to have the query return all documents in
  the index:

     U1db.Query {
          id: theQuery
          index: theIndex
      }

  So now the output becomes:

  JSON.stringify(theIndex)

  {"database":{"error":"","objectName":"","path":"indextest.db"},"name":"","objectName":"","expression":{"0":"title","1":"num"}}

  JSON.stringify(theQuery)

  {"index":{"database":{"error":"","objectName":"","path":"indextest.db"},"name":"","objectName":"","expression":{"0":"title","1":"num"}},"objectName":"","results":[{"title":"vic"},{"num":"123"}],"documents":{"0":"vic","1":"xqwzts"}}

  But now in order to add a query clause all the fields in the index
  which are also in the document have to be included.

      U1db.Query {
          id: theQuery
          index: theIndex
          query: [{"title": "*"}]
      }

  JSON.stringify(theQuery)

  {"index":{"database":{"error":"","objectName":"","path":"indextest.db"},"name":"","objectName":"","expression":{"0":"title","1":"num"}},"objectName":"","results":[{"title":"vic"}],"documents":{"0":"vic"},"query":[{"title":"*"}]}

  In order to actually return all the documents with title=* we have to
  also specify that num=*

      U1db.Query {
          id: theQuery
          index: theIndex
          query: [{"title": "*"}, {"num": "*"}]
      }

  JSON.stringify(theQuery)

  {"index":{"database":{"error":"","objectName":"","path":"indextest.db"},"name":"","objectName":"","expression":{"0":"title","1":"num"}},"objectName":"","results":[{"title":"vic"},{"num":"123"}],"documents":{"0":"vic","1":"xqwzts"},"query":[{"title":"*"},{"num":"*"}]}

  Conclusion:

  - In order to be included in the index, ALL fields in a document must be included in the index's expression clause.
  - In order to be returned by a query, ALL fields in a document [which are also in the index] must be included in the query's query clause.

  This is pretty problematic with large/complicated documents where
  including every single field in the expression/query clause is not
  really feasible.

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