Pattern for efficiently paging database rows on a web page

It shouldn’t be difficult to read rows from a database and render them on a web page – it’s a pretty fundamental operation that most web apps have to do but a surprising number don’t get quite right, especially when the volume of data grows and it can’t all be loaded and rendered within a single request. I’ve recently come across several ‘enterprise applications’ that simply fail to get it right – one extreme example loaded all the database rows into session state and then used LINQ to get and display the pages from that (!) and another took the approach of simply not paging the results at all which meant huge pages that were incredibly slow to load and as the data grew beyond the simple developer-test dataset.

So, I’m going to try and explain the technique I use for paging through rows which I’ve found works efficiently and is easy to re-use. It works well for both traditional and AJAX requests and handles sorting and filtering requirements as well. This is based on ASP.NET MVC, jQuery and NHibernate + SQL Server but the principals will be the same for other technologies (I’ve used the same basic approach with MongoDB as the back-end data-store).

Paged Data Model

We’ll start by defining a generic abstract class that will make it easy for us to re-use the approach for different lists. This will represent the ‘set’ of data on a page together with some additional information about the position of the page within the entire list, the total number of records and the sort order applied etc…

/// <summary>
/// Represents a paged list of data.
/// </summary>
/// <typeparam name="T">The item type in the list</typeparam>
/// <typeparam name="TS">The sort order specifier type.</typeparam>
public abstract class PagedList<T, TS>
{
    /// <summary>
    /// Gets or sets the list of entities.
    /// </summary>
    public IEnumerable<T> List { get; set; }

    /// <summary>
    /// Gets or sets the current page number.
    /// </summary>
    public int PageNo { get; set; }

    /// <summary>
    /// Gets or sets the number of records per page.
    /// </summary>
    public int PageSize { get; set; }

    /// <summary>
    /// Gets or sets the total number of records.
    /// </summary>
    public int RecordCount { get; set; }

    /// <summary>
    /// Gets or sets the first record number displayed.
    /// </summary>
    public int StartRecord { get; set; }

    /// <summary>
    /// Gets or sets a value indicating whether this <see cref="PagedList<T, TS>"/> is the latest page.
    /// </summary>
    /// <value><c>true</c> if latest; otherwise, <c>false</c>.</value>
    public bool Latest { get; set; }

    /// <summary>
    /// Gets or sets SortSeq.
    /// </summary>
    public TS SortSeq { get; set; }
}

The <T> and <TS> generic property types represent the type of entry that will be in the list (the ‘things’ we are paging over) and an Enum representing the sort-order required. As an example, we’ll page over a set of UserSummary entities and the sort-order will be defined with a UserSortSequence Enum:

/// <summary>
/// How users are ordered in result sets
/// </summary>
public enum UserSortSequence
{
    /// <summary>
    /// Order by name alphabetically
    /// </summary>
    ByName, 

    /// <summary>
    /// Order by date joined
    /// </summary>
    ByJoined, 

    /// <summary>
    /// Order by number of topics started
    /// </summary>
    ByTopics, 

    /// <summary>
    /// Order by number of posts
    /// </summary>
    ByPosts, 

    /// <summary>
    /// Order by reputation
    /// </summary>
    ByReputation
}

The PagedList is abstract so we need to create a specific class to represent the type of paged set that we want to use and this class will also contain any additional filtering parameters that our paging system will use. In this case, we will create a UserSet class that contains an additional Name property for filtering.

/// <summary>
/// Represents a user set.
/// </summary>
public class UserSet : PagedList<UserSummary, UserSortSequence>
{
    /// <summary>
    /// Gets or sets Name to filter.
    /// </summary>
    public string Name { get; set; }
}

Controller & Action

With these classes in place we can create a controller action to handle the request. A simplified version is shown below:

/// <summary>
/// Display paged user set filtered and sorted as required.
/// </summary>
/// <param name="name">The name filter.</param>
/// <param name="page">The page number.</param>
/// <param name="size">The page size.</param>
/// <param name="sort">The sort sequence.</param>
/// <returns></returns>
public ActionResult Index(string name, int page, int size, UserSortSequence sort)
{
    var model = new UserSet { PageNo = page, PageSize = size, SortSeq = sort, Name = name };
    this.Repository.PopulateUserSet(model);

    return View(model);
}

Data Access Layer

The real work is done in the PopulateUserSet method of the repository. This needs to do 3 things:

  1. Get the total number of rows matching whatever filter criteria are specified.
  2. Calculate the rows to display based on the page number and page size specified.
  3. Get the set of data from the database applying any sort order specified.

The sample below uses several NHibernate features and takes advantage of it’s ability to translate a paged-set request into the specific dialect of the database in question using whatever ROW_NUMBER functionality may be available so that the query operates as efficiently as possible.

/// <summary>
/// Populate user set with paged data.
/// </summary>
/// <param name="userset">The user set.</param>
public void PopulateUserSet(UserSet userset)
{
    // Get total count for all users in this set
    userset.RecordCount = this.session.CreateCriteria(typeof(Account))
        .Add(Restrictions.Like("Name", userset.Name, MatchMode.Anywhere))
        .SetProjection(Projections.RowCount())
        .UniqueResult<int>();

    // calculate the last page based on the record count and page size
    int lastPage = ((userset.RecordCount - 1) / userset.PageSize) + 1;

    // ensure page number is in range
    if (userset.PageNo < 1) {
        userset.PageNo = 1;
    }
    else if (userset.PageNo > lastPage) {
        userset.PageNo = lastPage;
        userset.Latest = true;
    }

    userset.StartRecord = (userset.PageNo - 1) * userset.PageSize;

    // create criteria to get user account with paging
    ICriteria userListCriteria = this.session.CreateCriteria(typeof(UserSummary))
        .Add(Restrictions.Like("Name", userset.Name, MatchMode.Anywhere))
        .SetFirstResult(userset.StartRecord)
        .SetMaxResults(userset.PageSize);

    // add ordering to criteria
    switch (userset.SortSeq)
    {
        case UserSortSequence.ByJoined:
            userListCriteria.AddOrder(Order.Asc("RegisteredOn"));
            break;
        case UserSortSequence.ByName:
            userListCriteria.AddOrder(Order.Asc("Name"));
            break;
        case UserSortSequence.ByPosts:
            userListCriteria.AddOrder(Order.Desc("PostCount"));
            break;
        case UserSortSequence.ByReputation:
            userListCriteria.AddOrder(Order.Desc("Reputation"));
            break;
        case UserSortSequence.ByTopics:
            userListCriteria.AddOrder(Order.Desc("TopicCount"));
            break;
    }

    // get the list of users
    userset.List = userListCriteria.List<UserSummary>();
}

View Rendering

So, we have a representation of a paged set of data, the action to retrieve it and the repository method to populate it. We now need a way of displaying the data and providing links for the user to navigate around it. How the rows are rendered obviously depends on the requirements of your application.

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage" %>
    userSummary) %>
        $(function() {
            setPaging('', , , );
        });

        function setPaging(name, page, pageSize, records) {
            $('.paging').paging({
                page: page,
                pageSize: pageSize,
                records: records,
                url: ''.replace('***', name).replace('99999', '#')
            });
        }

Rendering Paged Links

I’m using a jQuery plug-in to display the page links because it saves doing so much work on the server and it makes it easier to AJAX enable the UI. In the full implementation the controller action can return a full view or just the JSON data for the paged set and the UI uses a template to render it and update the page links.

The paging itself is rendered using a jQuery plug-in. This is designed to intelligently decide which links to render and whether to enable or disable the previous and next links. It keeps the first and last links always visible and ensures that a few links either side of the current page (which is highlighted) are rendered while avoiding silly gaps (like having 1, 2, … 4 instead of 1, 2, 3, 4). Also, the ‘…’ separators are also links to the midpoint of the non-displayed sequence to make it easier to quickly jump around even large lists.

Here is an example of a 10 page set of links rendering using the rules outlines above:

page-links

The code to render this is below:

/*
 * jQuery paging plugin
 *
 * http://www.captaincodeman.com/
 *
 */
(function($) {
    $.fn.paging = function(options) {

        // extend our default options with those provided.
        var settings = $.extend(true, {}, $.fn.paging.defaults, options);

        return this.each(function() {

            function render() {
                panel.empty();

                // calc pages, separators etc... to output
                var pageCount = Math.floor((settings.records - 1) / settings.pageSize) + 1;

                var pages = [];
                var pageStart, pageFinish;

                if (settings.inline) {
                    settings.prev = false;
                    settings.next = false;
                }

                var prevNextCount = 0;
                if (settings.prev) prevNextCount++;
                if (settings.next) prevNextCount++;

                if (settings.prev) {
                    if (currentPage > 1) {
                        pages[pages.length] = new PageEntry(settings.prevText, currentPage - 1, PageType.Previous);
                    } else {
                        pages[pages.length] = new PageEntry(settings.prevText, currentPage, PageType.PreviousDisabled);
                    }
                }

                if (pageCount <= settings.max + (settings.min * 2) + prevNextCount) {
                    // no separator required
                    addPages(pages, 1, pageCount);
                } else
                    if (currentPage <= settings.max) {
                    // cluster at the start
                    addPages(pages, 1, Math.max(currentPage + 1, settings.max));
                    addSeparator(pages, Math.max(currentPage + 1, settings.max), pageCount - settings.min);
                    addPages(pages, pageCount - settings.min + 1, pageCount);
                } else
                    if (currentPage >= pageCount - settings.max + 1) {
                    // cluster at the end
                    addPages(pages, 1, settings.min);
                    addSeparator(pages, settings.min, Math.min(currentPage - 1, pageCount - settings.max + 1));
                    addPages(pages, Math.min(currentPage - 1, pageCount - settings.max + 1), pageCount);
                } else {
                    // cluster in the middle
                    var offset = (settings.max - 1) / 2;
                    addPages(pages, 1, settings.min);
                    addSeparator(pages, settings.min, currentPage - offset);
                    addPages(pages, currentPage - offset, currentPage + offset);
                    addSeparator(pages, currentPage + offset, pageCount - settings.min + 1);
                    addPages(pages, pageCount - settings.min + 1, pageCount);
                }

                if (settings.next) {
                    if (currentPage < pageCount) {
                        pages[pages.length] = new PageEntry(settings.nextText, currentPage + 1, PageType.Next);
                    } else {
                        pages[pages.length] = new PageEntry(settings.nextText, currentPage, PageType.NextDisabled);
                    }
                }

                // render pages
                for (var idx = 0; idx < pages.length; idx++) {
                    if (settings.inline && pages[idx].Page == 1) {
                    } else {

                        var clickHandler = function(page) {
                            return function(event) { return pageSelected(UrlFormat(page), page, event); }
                        }

                        var item;

                        switch (pages[idx].PageType) {
                            case PageType.Previous:
                            case PageType.Next:
                                item = $("<a />")
                                    .bind("click", clickHandler(pages[idx].Page))
						            .attr("href", UrlFormat(pages[idx].Page))
						            .attr("class", 'prevnext')
						            .attr("title", "page " + pages[idx].Page)
						            .text(pages[idx].Text);
                                break;
                            case PageType.PreviousDisabled:
                            case PageType.NextDisabled:
                                item = $("<span />")
						            .attr("class", 'prevnext')
						            .text(pages[idx].Text);
                                break;
                            case PageType.Separator:
                                item = $("<a />")
                                    .bind("click", clickHandler(pages[idx].Page))
						            .attr("href", UrlFormat(pages[idx].Page))
						            .attr("class", 'sep')
						            .attr("title", "page " + pages[idx].Page)
						            .text(pages[idx].Text);
                                break;
                            case PageType.Page:
                                item = $("<a />")
                                    .bind("click", clickHandler(pages[idx].Page))
						            .attr("href", UrlFormat(pages[idx].Page))
						            .attr("title", "page " + pages[idx].Page)
						            .text(pages[idx].Text);
                                break;
                            case PageType.PageSelected:
                                item = $("<span />")
						            .attr("class", 'current')
						            .text(pages[idx].Text);
                                break;
                        }

                        panel.append(item);
                    }
                };
            }

            function pageSelected(url, page, event) {
                currentPage = page;
                var continuePropagation = settings.callback(url, page);
                if (!continuePropagation) {
                    if (event.stopPropagation) {
                        event.stopPropagation();
                    }
                    else {
                        event.cancelBubble = true;
                    }
                    render();
                }
                return continuePropagation;
            }

            function addPages(pages, start, finish) {
                for (var page = start; page <= finish; page++) {
                    if (page == currentPage) {
                        pages[pages.length] = new PageEntry(page, page, PageType.PageSelected);
                    } else {
                        pages[pages.length] = new PageEntry(page, page, PageType.Page);
                    }
                }
            }

            function addSeparator(pages, start, finish) {
                var page = Math.ceil((finish - start) / 2) + start;
                pages[pages.length] = new PageEntry(settings.separatorText, page, PageType.Separator);
            }

            function UrlFormat(page) {
                return settings.url.replace("#", page);
            }

            var panel = jQuery(this);
            var currentPage = settings.page;
            render();
        });
    };

    var PageType = {
        Previous: 3,
        PreviousDisabled: 4,
        Next: 5,
        NextDisabled: 6,
        Separator: 7,
        Page: 8,
        PageSelected: 9
    }

    function PageEntry(text, page, pageType) {
        this.Text = text;
        this.Page = page;
        this.PageType = pageType;
    }

    $.fn.paging.defaults = {
        page: 1,
        pageSize: 20,
        records: 0,
        min: 1,
        max: 3,
        inline: false,
        prev: true,
        prevText: '« prev',
        next: true,
        nextText: 'next »',
        separator: true,
        separatorText: '...',
        url: '#',
        callback: function() { return true; }
    };

})(jQuery);

Summary

These are all the pieces and hopefully it gives you an idea of how it all works. A full implementation will usually contain some extra pieces – besides the AJAX enabled rendering using JSON and client-side templates I’ll also have other links to enable to page-size to be adjusted and the sort-order changed and these are persisted to a cookie in the controller action using a custom binder. Hopefully though, what I’ve shown here will help you get a good solid and efficient paging system implemented.

Please let me know if any part of this is unclear or you’d like further details on any of the pieces.

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