Cursor-based paging for connections with MS SQL CTEs

_readme.md

Cursor-based paging with MS SQL with varying sort ordering

Uses SqlKata.

EF Core support:

• Discussion: dotnet/efcore#20967

fwd-query-1.sql

-- Connection args:
-- first: 10
WITH [cte] 
AS 
(
  SELECT 
    BookingID AS [Cursor],
    ROW_NUMBER() OVER (ORDER BY ServiceDate DESC, BookingID DESC) AS [RowNumber] 
  FROM [Booking] 
  -- WHERE {filters}
)
SELECT 
  [Cursor], 
  [RowNumber],
  (SELECT COUNT(*) FROM cte) AS TotalCount 
FROM [cte] 
WHERE [RowNumber] <= 10 -- limits to first x

-- client can determine:
-- totalCount = results.Any() ? results.First().TotalCount : 0;
-- hasNextPage = totalCount > 0 ? results.First().RowNumber != 1;
-- hasNextPage = totalCount > 0 && results.Last().RowNumber != totalCount;

fwd-query-2.sql

-- Connection args:
-- first: 10
-- after: 173391 (last cursor of previous query)
WITH [cte] 
AS 
(
  SELECT 
    BookingID AS RowCursor, 
    ROW_NUMBER() OVER (ORDER BY ServiceDate DESC, BookingID DESC) AS RowNumber
  FROM [Booking] 
  --WHERE {filters}
)
SELECT 
  [RowCursor], 
  [RowNumber], 
  (SELECT COUNT(*) FROM cte) AS TotalCount 
FROM [cte] 
WHERE RowNumber > ( -- limits to rows after last of previous query
  SELECT RowNumber 
  FROM [cte] 
  WHERE RowCursor = 173391
) 
AND RowNumber <= ( -- limts to first x rows after last or previous query
  SELECT [RowNumber] + 10 
  FROM [cte] 
  WHERE RowCursor = 173391
)

SqlKataExample.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Threading.Tasks;
using AutoGuru.Client.Shared;
using AutoGuru.Client.Shared.Dtos;
using AutoGuru.Client.Shared.Models;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.Logging;
using SqlKata;
using SqlKata.Execution;

namespace AutoGuru.Service.Shared.Querying
{
    public class ConnectionPager
    {
        private readonly ILogger<ConnectionPager> _logger;
        private readonly IConfiguration _configuration;
        private const string CTE = "cte";
        private const string RowCursor = "RowCursor";
        private const string RowNumber = "RowNumber";

        public ConnectionPager(ILogger<ConnectionPager> logger, IConfiguration configuration)
        {
            _logger = logger;
            _configuration = configuration;
        }

        public async Task<TConnectionDto> GetConnectionAsync<TCursor, TNodeDto, TEdgeDto, TConnectionDto>(
            ConnectionPaginationArguments pagingArgs,
            QueryFactory db,
            Query query,
            string cursorColumnName,
            Func<TNodeDto, TCursor> getCursorFunc,
            Func<List<TCursor>, Task<TNodeDto[]>> getItemsAsyncFunc)
            where TConnectionDto : ConnectionDto<TNodeDto, TEdgeDto>, new()
            where TEdgeDto : EdgeDto<TNodeDto>, new()
        {
            var cteQuery = query.Clone();

            // Get the order by, cursor-determining expression
            var order = db.Compiler
                .CompileOrders(new SqlResult
                {
                    Query = cteQuery
                });
            
            if (string.IsNullOrWhiteSpace(order))
            {
                throw new Exception($"{nameof(query)} does not have an ORDER BY clause");
            }

            // Build a CTE from given query to form a dictionary of { RowCursor, RowNumber }
            var shouldSelectTotalCount = pagingArgs.IsTotalCountRequested || pagingArgs.IsHasPrevPageRequested || pagingArgs.IsHasNextPageRequested;
            cteQuery.Clauses.RemoveAll(c => c.Component == "order");
            cteQuery.Clauses.RemoveAll(c => c.Component == "select");
            cteQuery.SelectRaw(
                $"{cursorColumnName} AS {RowCursor}, " +
                $"ROW_NUMBER() OVER ({order}) AS {RowNumber}");

            // Build an edges query using that CTE's dictionary to slice out the records we need leveraging RowNumber
            var edgesQuery = db.Query()
                .With(CTE, cteQuery)
                .From(CTE)
                .SelectRaw(
                    $"{RowCursor}, " +
                    $"{RowNumber}" +
                    (shouldSelectTotalCount ? $", (SELECT COUNT(*) FROM {CTE}) AS TotalCount" : ""));

            // Filter by all rows after or for given after cursor
            var hasAfterCursor = !string.IsNullOrWhiteSpace(pagingArgs.After);
            var afterCursor = hasAfterCursor ? Cursor.FromCursor<TCursor>(pagingArgs.After) : default;
            if (hasAfterCursor)
            {
                edgesQuery.Where(RowNumber, ">",
                    new Query(CTE)
                        .Select(RowNumber)
                        .Where(RowCursor, afterCursor));
            }

            // Filter by all rows before or for given before cursor
            var hasBeforeCursor = !string.IsNullOrWhiteSpace(pagingArgs.Before);
            var beforeCursor = hasBeforeCursor ? Cursor.FromCursor<TCursor>(pagingArgs.Before) : default;
            if (hasBeforeCursor)
            {
                edgesQuery.Where(RowNumber, "<",
                    new Query(CTE)
                        .Select(RowNumber)
                        .Where(RowCursor, beforeCursor));
            }

            // Select the first x amount of rows
            if (pagingArgs.First.HasValue)
            {
                // If the after cursor is defined
                if (hasAfterCursor)
                {
                    edgesQuery.Where(RowNumber, "<=",
                        new Query(CTE)
                            .SelectRaw($"{RowNumber} + {pagingArgs.First.Value}")
                            .Where(RowCursor, afterCursor));
                }
                // If no after cursor is defined
                else
                {
                    edgesQuery.Where(RowNumber, "<=", pagingArgs.First.Value);
                }
            }
            else // Select the last x amount of rows
            {
                // If the before cursor is defined
                if (hasBeforeCursor)
                {
                    edgesQuery.Where(RowNumber, ">=",
                        new Query(CTE)
                            .SelectRaw($"{RowNumber} - {pagingArgs.Last.Value}")
                            .Where(RowCursor, beforeCursor));
                }
                // If we have to take data all the way from the back
                else
                {
                    edgesQuery.Where(RowNumber, ">",
                        new Query(CTE)
                            .SelectRaw($"COUNT(*) - {pagingArgs.Last.Value}"));
                }
            }

            // Execute the edges query to determine the records we need (ids),
            // total count and has next/prev page if requested

            dynamic[] pageOfEdges;
            try
            {
                var sw = Stopwatch.StartNew();
                pageOfEdges = (await edgesQuery.GetAsync()).ToArray();

                if (_logger.IsEnabled(LogLevel.Information))
                {
                    // Wrapped in a log level check so we're not compiling the query unnecessarily
                    _logger.LogInformation("Executed DbCommand (SqlKata) ({elapsed}ms) {commandText}",
                        sw.ElapsedMilliseconds,
                        db.Compiler.Compile(edgesQuery));
                }
            }
            catch (Exception ex)
            {
                // For debugging, you can get the generated SQL query here
                var sqlResult = db.Compiler.Compile(edgesQuery);

                _logger.LogError(ex, $"Error performing Connection DB query for page of edges. Query = {sqlResult.Sql}");
                throw;
            }

            var totalCount = shouldSelectTotalCount && pageOfEdges.Any() ? (int)pageOfEdges.First().TotalCount : 0;
            var hasPrevPage = totalCount > 0 && pageOfEdges.First().RowNumber != 1;
            var hasNextPage = totalCount > 0 && pageOfEdges.Last().RowNumber != totalCount;
            var cursors = pageOfEdges.Select(e => (TCursor)e.RowCursor).ToList();

            // Execute the given EF query func to get the DTOs we need
            TNodeDto[] edges;
            try
            {
                edges = (await getItemsAsyncFunc(cursors))
                    .OrderBy(i => cursors.IndexOf(getCursorFunc(i)))
                    .ToArray();
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "Error performing Connection DB query for full DTOs.");
                throw;
            }

            // Return as a connection dto
#pragma warning disable IDE0039 // Use local function, doesn't compile when local function
            Func<TNodeDto, object> getCursorFunc2 = i => getCursorFunc(i);
#pragma warning restore IDE0039 // Use local function

            return ConnectionDto.From<TNodeDto, TEdgeDto, TConnectionDto>(
                edges,
                getCursorFunc2,
                hasPrevPage,
                hasNextPage,
                totalCount
            );
        }
    }
}