检查专用 SQL 池的统计信息准确性
适用对象：Azure Synapse Analytics
更新的统计信息对于生成最佳执行计划至关重要。 在确定统计信息准确性时，应从两个不同的角度进行评估：
步骤 1：验证控制节点行计数准确性
在专用 SQL 池中，需要更新用于创建分布式查询计划的主引擎，了解计算节点上的行数。 运行以下查询以识别行计数之间存在差异的表：
SELECT objIdsWithStats.[object_id]
,actualRowCounts.[schema]
,actualRowCounts.logical_table_name
,statsRowCounts.stats_row_count
,actualRowCounts.actual_row_count
,row_count_difference = CASE
WHEN actualRowCounts.actual_row_count >= statsRowCounts.stats_row_count
THEN actualRowCounts.actual_row_count - statsRowCounts.stats_row_count
ELSE statsRowCounts.stats_row_count - actualRowCounts.actual_row_count
END
,percent_deviation_from_actual = CASE
WHEN actualRowCounts.actual_row_count = 0
THEN statsRowCounts.stats_row_count
WHEN statsRowCounts.stats_row_count = 0
THEN actualRowCounts.actual_row_count
WHEN actualRowCounts.actual_row_count >= statsRowCounts.stats_row_count
THEN CONVERT(NUMERIC(18, 0), CONVERT(NUMERIC(18, 2), (actualRowCounts.actual_row_count - statsRowCounts.stats_row_count)) / CONVERT(NUMERIC(18, 2), actualRowCounts.actual_row_count) * 100)
ELSE CONVERT(NUMERIC(18, 0), CONVERT(NUMERIC(18, 2), (statsRowCounts.stats_row_count - actualRowCounts.actual_row_count)) / CONVERT(NUMERIC(18, 2), actualRowCounts.actual_row_count) * 100)
END
,'UPDATE STATISTICS ' + quotename(actualRowCounts.[schema]) + '.' + quotename(actualRowCounts.logical_table_name) + ';' as update_stats_stmt
FROM (
SELECT DISTINCT object_id
FROM sys.stats
WHERE stats_id > 1
) objIdsWithStats
LEFT JOIN (
SELECT object_id
,sum(rows) AS stats_row_count
FROM sys.partitions
GROUP BY object_id
) statsRowCounts ON objIdsWithStats.object_id = statsRowCounts.object_id
LEFT JOIN (
SELECT sm.name [schema]
,tb.name logical_table_name
,tb.object_id object_id
,SUM(rg.row_count) actual_row_count
FROM sys.schemas sm
INNER JOIN sys.tables tb ON sm.schema_id = tb.schema_id
INNER JOIN sys.pdw_table_mappings mp ON tb.object_id = mp.object_id
INNER JOIN sys.pdw_nodes_tables nt ON nt.name = mp.physical_name
INNER JOIN sys.dm_pdw_nodes_db_partition_stats rg ON rg.object_id = nt.object_id
AND rg.pdw_node_id = nt.pdw_node_id
AND rg.distribution_id = nt.distribution_id
INNER JOIN sys.indexes ind on tb.object_id = ind.object_id
WHERE rg.index_id < 2 -- In case this condition removed the number of rows will gets duplicated based on the number of index.
AND ind.type_desc IN ('CLUSTERED COLUMNSTORE', 'HEAP') -- Switch between the CCI (Column store) and HEAP, You should at least keep one value or else the total number of rows will gets duplicated based on the number of indexes.
GROUP BY sm.name
,tb.name
,tb.object_id
) actualRowCounts ON objIdsWithStats.object_id = actualRowCounts.object_id
步骤 2：确保统计信息是最新的
更新数据可能会显著影响用于生成有效执行计划的统计信息直方图。 运行以下查询，确定统计信息的上次更新日期是否与表的修改模式一致：
SELECT ob.[object_id],max(sm.[name]) AS [schema_name]
,max(tb.[name]) AS [table_name]
,st.[stats_id]
,max(st.[name]) AS [stats_name]
,string_agg(co.[name], ',') AS [stats_column_names]
,STATS_DATE(ob.[object_id], st.[stats_id]) AS [stats_last_updated_date]
,'UPDATE STATISTICS ' + quotename(max(sm.[name])) + '.' + quotename(max(tb.[name])) + ';' as [update_stats_stmt]
FROM sys.objects ob
JOIN sys.stats st ON ob.[object_id] = st.[object_id]
JOIN sys.stats_columns sc ON st.[stats_id] = sc.[stats_id]
AND st.[object_id] = sc.[object_id]
JOIN sys.columns co ON sc.[column_id] = co.[column_id]
AND sc.[object_id] = co.[object_id]
JOIN sys.types ty ON co.[user_type_id] = ty.[user_type_id]
JOIN sys.tables tb ON co.[object_id] = tb.[object_id]
JOIN sys.schemas sm ON tb.[schema_id] = sm.[schema_id]
WHERE st.[stats_id] > 1
GROUP BY ob.[object_id], st.[stats_id]
ORDER BY stats_last_updated_date
步骤 3：更新已识别的表上的统计信息
在前面的步骤中标识候选表后，运行在 update_stats_stmt 查询列中生成的语句以更新统计信息。
注意
建议不要更新单个统计信息，即使用户创建也是如此。 通过在不指定统计信息名称的情况下运行 UPDATE STATISTICS，可更新与表关联的所有统计信息以及控制节点行计数。 可以考虑通过使用 WITH FULLSCAN 或 WITH SAMPLE <SamplePercent> PERCENT 实现大型表的适当准确性来替代默认扫描百分比。 有关完整语法，请参阅 UPDATE STATISTICS (Transact-SQL)。
例如：
UPDATE STATISTICS [dbo].[MyAwesomeTable];
更新统计信息后，重新运行问题查询以确定统计信息更新是否缩短了执行持续时间。