Difference in query results when run from HANA MDX and HANA SQL
SAP HANA offers multiple access options to query SAP HANA views available in database:
- The HANA SQL access, the primary HANA access, allows you to query HANA views using the SQL query language. This access is optimized for on-premise solutions.
- The HANA MDX access, that offers multi-dimensional concepts to HANA models, allows you to query HANA views as cubes (including HANA hierarchies) through the Multi-Dimensional eXpressions (MDX) query language.
- The HANA InA access, designed for Web applications or solutions, allows you to provide real time access to information stored in HANA database using the SAP HANA Info Access (InA) HTTP Service. The SAP HANA info access HTTP service wraps SQL queries and exposes them through an HTTP interface.
This document describes the difference in query results when you run the same query specification on a same HANA view from HANA SQL/InA and HANA MDX.
Use case description
To illustrate this difference, let’s consider a HANA view (named FINANCE here) containing the following dimensions and measures:
| Type | Name | Label |
| Dimension | DIMACCOUNT | Account |
| Dimension | DIMDEPARTMENT | Department |
| Dimension | DIMORGANIZATION | Organization |
| Dimension | DIMSCENARIO | Scenario |
| Dimension | DIMTIME | Time |
| Measure | AMOUNT | Amount |
In this HANA view, you also create and associate the following HANA hierarchies to the dimensions:
| Dimension | Hierarchy Name | Hierarchy Type |
| DIMACCOUNT | ACCOUNTS | Parent-child hierarchy |
| DIMDEPARTMENT | DEPARTMENTS | Parent-child hierarchy |
| DIMTIME | DATE_CALENDAR | Level-based hierarchy |
| DIMTIME | FISCAL_CALENDAR | Level-based hierarchy |
In HANA SQL, to get the query results for Amount per Organization, you write the following script:
|
SELECT View__1.”ORGANIZATION_KEY”, View__1.”ORGANIZATION”, SUM(View__1.”AMOUNT”) FROM “_SYS_BIC”.”adventure-works/FINANCE” View__1 GROUP BY View__1.”ORGANIZATION_KEY”, View__1.”ORGANIZATION” ORDER BY 1
|
And get the following result:
| Organization key | Organization | Amount |
| 3 | Northeast Division | 134003346 |
| 4 | Northwest Division | 138922889 |
| 5 | Central Division | 149032082 |
| 6 | Southeast Division | 279284664 |
| 7 | Southwest Division | 232365971 |
| 8 | Canadian Division | 292174769 |
| 11 | France | 66963785 |
| 12 | Germany | 30339796 |
| 13 | Australia | 35553070 |
In HANA MDX, you write the following MDX script to express the same query specification:
|
SELECT NON EMPTY {[Measures].[AMOUNT]} ON COLUMNS, NON EMPTY [DIMORGANIZATION].[DIMORGANIZATION].[ORGANIZATION_KEY].Members DIMENSION PROPERTIES [DIMORGANIZATION].[DIMORGANIZATION].[ORGANIZATION_KEY].[ORGANIZATION] ON ROWS FROM [FINANCE]
|
As a result, you receive:
| Organization key | Organization | Amount |
| 3 | Northeast Division | 106108814 |
| 4 | Northwest Division | 108873980 |
| 5 | Central Division | 117558826 |
| 6 | Southeast Division | 252761627 |
| 7 | Southwest Division | 183205286 |
| 8 | Canadian Division | 231313761 |
| 11 | France | 54658581 |
| 12 | Germany | 24280245 |
| 13 | Australia | 28249786 |
The two queries return different results in the Amount values (Note that the HANA InA access provides the same query results as HANA SQL).
Why this difference between the SQL and MDX queries?
Paradigm difference between SQL and MDX
The query results displayed above are both corrects for HANA SQL and HANA MDX. However, SQL and MDX have different paradigms that need to be understood.
In the query specification above, you only want to query the Amount per Organization.
In the SQL paradigm, this means to ignore all other dimensions (i.e. Account, Department, Scenario and Time) and to just aggregate implicitly the Amount over them.
In the MDX world, the paradigm is different. Indeed, the MDX paradigm is like the cartesian (x, y) axis coordinates. Suppose you have a function f such as Amount = f (x, y) (i.e. Amount is a function of x and y). What would be the Amount for x = 10? It cannot be defined because Amount is a function of x and y and y is unknown. Amount cannot be determined since values for x and y are both required. The MDX standard works in this mode.
Thus, in MDX, for the HANA view defined above, the Amount measure depends on the dimensions Account, Department, Organization, Scenario and Time. Because your MDX query above includes explicitly only the Organization dimension to get Amount, the query result is therefore indeterminate.
For this use case, in MDX, when a dependent dimension is not explicitly included in a query axis to evaluate a measure, the default member of that dimension is implicitly included in the slicer axis to complete the coordinates (see https://msdn.microsoft.com/en-us/library/ms146047.aspx )
Determining the default member in SAP HANA
In HANA MDX, each dimension has a default hierarchy associated to it. These default hierarchies can be implicitly generated by HANA MDX; or authored in the HANA view by the designer. And each of these hierarchies may have a default member.
In HANA, the default member of a hierarchy is, by default, the member that has the property MEMBER_ORDINAL = 0.
For hierarchies implicitly generated by HANA MDX, the default member corresponds to the hierarchy root node i.e. the “ALL” member. However, for hierarchies authored in the HANA view, the default member depends on hierarchy settings.
To know the default member associated to hierarchies in your HANA view in MDX, you can run the following query:
| MDX SELECT “DIMENSION_UNIQUE_NAME”, “HIERARCHY_UNIQUE_NAME”, “DEFAULT_MEMBER” FROM BIMC_HIERARCHIES WHERE “CATALOG_NAME”=’adventure-works’ AND “CUBE_NAME”=’FINANCE’ |
That returns the following result:
| DIMENSION_UNIQUE_NAME | HIERARCHY_UNIQUE_NAME | DEFAULT_MEMBER |
| [DIMACCOUNT] | [DIMACCOUNT].[ACCOUNTS] | [DIMACCOUNT].[ACCOUNTS].&[1] |
| [DIMDEPARTMENT] | [DIMDEPARTMENT].[DEPARTMENTS] | [DIMDEPARTMENT].[DEPARTMENTS].&[1] |
| [DIMORGANIZATION] | [DIMORGANIZATION].[DIMORGANIZATION] | [DIMORGANIZATION].[DIMORGANIZATION].[All].[(all)] |
| [DIMSCENARIO] | [DIMSCENARIO].[DIMSCENARIO] | [DIMSCENARIO].[DIMSCENARIO].[All].[(all)] |
| [DIMTIME] | [DIMTIME].[DATE_CALENDAR] | [DIMTIME].[DATE_CALENDAR].[All].[(all)] |
| [DIMTIME] | [DIMTIME].[FISCAL_CALENDAR] | [DIMTIME].[FISCAL_CALENDAR].[All].[(all)] |
| [Measures] | [Measures].[Measures] | [Measures].[AMOUNT] |
In the query result above, you can observe that all hierarchies have a root node (i.e. the ALL member) as default member; excepted for the Accounts and Departments hierarchies that have respectively ACCOUNTS.&[1] and DEPARTMENTS.&[1] as default members.
This means that your MDX query above corresponds to the Amount for ACCOUNTS.&[1] and DEPARTMENT.&[1] i.e. the Amount will be aggregated over all SCENARIO and TIME dimension members (as in SQL) but it will be NOT aggregated over all ACCOUNTS or DEPARTMENTS.
That explains why the MDX query result for Amount is less than the one you observe in SQL – but this is correct.
Solution
The HANA modelers (HANA Studio or Web IDE For SAP HANA) offer various ways to set explicitly or implicitly default members for HANA hierarchies authored in HANA views.
To produce the same query results when run from HANA SQL and HANA MDX, the solution consists of having authored hierarchies with default members that place all other hierarchy members as their descendants.
The simplest workaround is to add a root node as default member for your authored hierarchies:
- Edit your hierarchy
- Go to Advanced settings
- Clear the Default Member field (if non-empty)
- Set “Add Root Node” for the Root Node Visibility option
Indeed, when you set Add Root Node for the Root Node Visibility option, then an ALL member is added as default member if the Default Member field is empty. For parent-child hierarchies, if you set Add Root Node If Defined for the Root Node Visibility option, then a root node is added only if you have defined a root node value while creating the parent child hierarchy.
Thus, if you set these options for ACCOUNTS and DEPARTMENTS hierarchies, then your MDX query returns the same result as your SQL query.
Have fun with HANA!
Is there any restriction to push down the calculation to DB server in HANA SQL/InA and HANA MDX.
Hi,
No, there is not particular restriction or setting to push down the calculation to DB server in HANA SQL/InA and HANA MDX. However, the difference between SQL and MDX paradigms needs to be well understood for a better use according to your business cases
Best regards,
Hi Olivier,
You have mentioned that:
The SAP HANA Developer Guide describes the features of Information Access (InA) Queries in the context of XSA applications. This seems like a great way to build XSA applications that need to access analytical queries (based on Calculations Views or CDS Views for example). However, I am unable to find any API documentation that explains the syntax of the Request parameter of the InA service. Can you provide any guidance about any available InA API documentation?
Thanks,
Mustafa.
Hi,
For the moment, the InA protocol is an internal SAP protocol. That explains why a public InA API documentation is not yet avalaible. A public InA API for OEMs is already in the SAP backlog.
Best regards,
Thanks Olivier. It’s good to know that a public InA API is in the pipeline.
Respectfully – you didn’t write the same query. That is, “the same query”, to me means: “equivalent”. If the query results are not the same, then you didn’t write equivalent queries.
It’s als not the case that the MDX query returns an “indeterminate” result. “Indeterminate” is when, given the same data, filters, environment variables and so on, the result can still vary. Clearly, neither query is “indeterminate”: as long as the data and the view definition remain unchanged, you can execute either query as often as you like, and the result would always remain stable.
What you probably meant to say is that in MDX the query result is implicitly bound by the default members on the hierarchies that are not included explicitly in the query.
(We can have effects like this too in SQL queries, just think of how some queries have a different result depending on the value of a session variable, like CURRENT_USER).
Indeterminate would be if your view contained some function like RAND(), or CURRENT_TIMESTAMP, or if it would select from a sequence or something like that.
The piece about controlling the default members is a good tip. But it’s worth pointing out that it would have been possible to write the MDX query in an equivalent way by adding a slicer consising of the set of all members at the topmost level of each hierarchy that does not appear on any of the axes in the SELECT clause.
Often hierarchies have an “All” member that makes this easier, so for example, iby adding
WHERE ([DIMACCOUNT].[ACCOUNTS].[(all)], [DIMDEPARTMENT].[DEPARTMENTS].[(all)], [DIMSCENARIO].[DIMSCENARIO].[(all]), [DIMTIME].[DATE_CALENDAR].[All].[(all)], [DIMTIME].[FISCAL_CALENDAR].[All].[(all)])
the MDX query could have been writtien in a truly equivalent way, without modifying the definition of the view.
Hi Roland,
You are right when you simply say: "In MDX the query result is implicitly bound by the default members on the hierarchies that are not included explicitly in the query". That is what mainly differentiates the MDX with SQL.
Indeed, as solution, we can get same query results in MDX (like in SQL) in adding the 'ALL' members of missing hierarchies in the SLICER axis.
Best regards,
Excel Pivot tables use MDX as query language to read data from SAP HANA system. MDX is defined as part of ODBO (OLE DB for OLAP) specification from Microsoft and is used for data selections, calculations and layout.
SAP SQL Anywhere is a proprietary relational database management system (RDBMS) product from SAP. SQL Anywhere was known as Sybase SQL Anywhere prior to the acquisition of Sybase by SAP.
Hi,
Thanks for sharing this.
I’m working with SAP HANA 2.0 SPS3 and SAP BI 4.2 SP6. Web Intelligence is connecting directly with HANA view thanks to OLAP Connection created from CMC.
We have differences in query result between Web Intelligence and Hana Studio (or Analysis for OLAP)
For exemple :
When I run the mdx query I get from this blog, I see that dimensions has ...[All] as default member.
Can you please advice ?
Hi,
it is hard to provide global answers because the roots of these cases are multiples and they can require to analyze the specified views.
From HANA OLAP, some dimensions can return empty data in WebI. You can enable the "Retreive empty rows" query option in the query panel to check if you get all dimension members.
In HANA MDX, the hierarchies implicitly generated by HANA have a default member that corresponds to the “ALL” member. If you want to change it, you must explicitly create your hierarchy and set your own default member.
Regards,
Olivier T.
Thanks