indexedDB中的IN运算符对等 [英] IN Operator equivalence in indexedDB

问题描述

我要执行此查询从propertyCode IN("field1"，"field2"，"field3")的属性中选择*

I want to execute this query select * from properties where propertyCode IN ("field1", "field2", "field3")

如何在IndexedDB中实现这一目标我尝试过这个东西

How can I achieve this in IndexedDB I tried this thing

getData : function (indexName, params, objectStoreName) {
            var defer = $q.defer(),
                db, transaction, index, cursorRequest, request, objectStore, resultSet, dataList = [];
            request = indexedDB.open('test');
            request.onsuccess = function (event) {
                db = request.result;
                transaction = db.transaction(objectStoreName);
                objectStore = transaction.objectStore(objectStoreName);
                index = objectStore.index(indexName);
                cursorRequest = index.openCursor(IDBKeyRange.only(params));
                cursorRequest.onsuccess = function () {

                    resultSet = cursorRequest.result;
                    if(resultSet){
                        dataList.push(resultSet.value);
                        resultSet.continue();
                    }
                    else{
                        console.log(dataList);
                        defer.resolve(dataList);
                    }
                };

                cursorRequest.onerror = function (event) {
                    console.log('Error while opening cursor');
                }
            }
            request.onerror = function (event) {
                console.log('Not able to get access to DB in executeQuery');
            }
            return defer.promise;

但是没有用.我尝试使用Google，但找不到确切答案.

But didn't worked. I tried google but couldn't find exact answer.

推荐答案

如果您认为IN基本上等于 field1 == propertyCode或field2 == propertyCode ，那么您可以说IN是只是使用OR的另一种方式.

If you consider that IN is essentially equivalent to field1 == propertyCode OR field2 == propertyCode, then you could say that IN is just another way of using OR.

IndexedDB无法对单个请求执行OR(联合).

IndexedDB cannot do OR (unions) from a single request.

通常，您唯一的选择是执行单独的请求，然后将其合并到内存中.通常，这不会有很好的性能.如果您要处理许多对象，则可能要考虑完全放弃这种方法，并思考如何避免这种方法.

Generally, your only recourse is to do separate requests, then merge them in memory. Generally, this will not have great performance. If you are dealing with a lot of objects, you might want to consider giving up altogether on this approach and thinking of how to avoid such an approach.

另一种方法是遍历内存中的所有对象，然后过滤不符合您条件的对象.再次，糟糕的表现.

Another approach is to iterate over all objects in memory, and then filter those that don't meet your conditions. Again, terrible performance.

这是个花哨的技巧，可能会给您带来不错的性能，但它需要一些额外的工作和少量的存储开销:

Here is a gimmicky hack that might give you decent performance, but it requires some extra work and a tiny bit of storage overhead:

• 在您的对象中存储一个额外的字段.例如，计划使用名为 hasPropertyCodeX 的属性.
• 这3个属性中的任何一个为true(具有正确的代码)时，请设置字段(例如，仅使其成为对象的属性，则其值无关紧要).
• 如果这三个属性都不为真，则从对象中删除该属性.
• 只要修改了对象，就更新派生的属性(适当地设置或取消设置它).
• 在indexedDB中的此派生属性上创建索引.
• 在索引上打开光标.只有具有属性的对象才会出现在光标结果中.

var request = indexedDB.open(...);
request.onupgradeneeded = upgrade;

function upgrade(event) {
  var db = event.target.result;
  var store = db.createObjectStore('store', ...);

  // Create another index for the special property
  var index = store.createIndex('hasPropCodeX', 'hasPropCodeX');
}

function putThing(db, thing) {

  // Before storing the thing, secretly update the hasPropCodeX value
  // which is derived from the thing's other properties
  if(thing.field1 === 'propCode' || thing.field2 === 'propCode' || 
    thing.field3 === 'propCode') {
    thing.hasPropCodeX = 1;
  } else {
    delete thing.hasPropCodeX;
  }

  var tx = db.transaction('store', 'readwrite');
  var store = tx.objectStore('store');
  store.put(thing);
}

function getThingsWherePropCodeXInAnyof3Fields(db, callback) {
  var things = [];
  var tx = db.transaction('store');
  var store = tx.objectStore('store');
  var index = store.index('hasPropCodeX');
  var request = index.openCursor();
  request.onsuccess = function(event) {
    var cursor = event.target.result;
    if(cursor) {
      var thing = cursor.value;
      things.push(thing);
      cursor.continue();
    } else {
      callback(things);
    }
  };
  request.onerror = function(event) {
    console.error(event.target.error);
    callback(things);
  };
}

// Now that you have an api, here is some example calling code
// Not bothering to promisify it
function getData() {
  var request = indexedDB.open(...);
  request.onsuccess = function(event) {
    var db = event.target.result;
    getThingsWherePropCodeXInAnyof3Fields(db, function(things) {
      console.log('Got %s things', things.length);
      for(let thing of things) {
        console.log('Thing', thing);
      }
    });
  };
}

这篇关于indexedDB中的IN运算符对等的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！