Edge Cases in Streaming Operation

As explained in the section Streaming Vs Non-Streaming Operators, when the data is combined using exclusively streaming operators (such as merge and nested joins, unions, selections, projections, etc.), the memory consumption of the query should be minimal, no matter how many rows need to be processed by Denodo. Nevertheless, there are some edge cases where memory consumption can be significant:

  1. Non-streaming data sources. Some data sources do not support returning data in “streaming mode”. In these cases, Denodo has to store in memory all the results returned by the data source. If the returned number of rows is high, this can result in significant memory consumption. Most data sources that can return large amounts of data (e.g. databases) support streaming in a way or another to avoid forcing their client applications to store large result sets. To avoid memory overflows, ensure you use streaming always when possible.

  2. Large rows. In streaming operators, Denodo processes result sets row by row, so typically only a few rows need to be in memory at the same time. Nevertheless, if rows are extremely large, then the query still consumes a significant amount of memory. This may be the case when a row contains complex hierarchical data (remind that Denodo has the capability to represent complex hierarchical data using the array and the register data types).

    For instance, by default, the Denodo XML data source creates one single row containing the full result of a query (you can latter use the flatten operation to divide it into several rows). If you think a certain XML data service will return large documents, use the “Tuple Root” option in the view creation wizard to divide the document in several rows, avoiding this problem. The section XML Sources explains how to do this.

    There are similar options in other data sources where it is common to return complex hierarchical data, such as JSON web services and SOAP Web Services

  3. Slow data sources (or slow client applications). As explained in the section Streaming Vs Non-Streaming Operators, Denodo retrieves data from the data sources involved in a query in parallel. Suppose Denodo is joining two views A and B, and the data comes from the data sources already sorted by the join attributes (so the join can be performed using a streaming operator). Suppose also that the data source A is much faster than data source B. Then, the thread of the data source A will tend to accumulate rows in memory, since they cannot be processed until the matching rows of B arrive. To avoid the number of accumulated rows to grow excessively, Denodo limits the maximum amount of memory that can be used to accumulate rows waiting to be processed, through the “Maximum size in memory of intermediate results” parameter. This ensures that, no matter how slow is the other data source, memory usage will not grow out of control.

    By default, Denodo uses a relatively small value for the “Maximum size in memory of intermediate results” parameter, (the mechanism to limit the maximum memory consumed by a query explained in the section Limit the Maximum Amount of Memory of a Query can also automatically decrease it even more, if it thinks it is needed). In general, you do not need to modify this value, but increasing it can improve performance in very specific cases. Here is an example:

    1. Suppose we have a join view V between two views A (from data source DS1, which is a SaaS application like Salesforce) and B (from data source DS2, which is a database in the same network as the Virtual DataPort server).
    2. B pushes down a very complex aggregation query on the DS2 database and A executes a relatively simple query on DS1.
    3. Since B pushes down a complex aggregation query to DS2, suppose that DS2 does not begin returning results until 30 seconds after the query was executed (the time required by the database to compute the results). After the first results are returned, the remaining results will come very fast, since the database is in the same network as the Virtual DataPort server.
    4. In turn, the results from DS1 will start to come almost immediately (since the query executed on DS1 is simple) but they will arrive at a relatively slow pace because they come from an external network.
    5. In these circumstances, increasing the value of the “Maximum size in memory of intermediate results” will result in a better execution time: the extra space will allow Denodo to retrieve results from DS1 during the 30 seconds that DS2 is busy computing the query, even if they are still not needed, so they are ready when data from DS2 finally arrives. In this case, if the “Maximum size in memory of intermediate results” parameter has a small value, then the available space would fill soon, and Denodo would spend most of the 30 first seconds just waiting for DS2, without doing actual work.