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pg_stat_statements

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    Alvaro Herrera authored
    XMLTABLE is defined by the SQL/XML standard as a feature that allows
    turning XML-formatted data into relational form, so that it can be used
    as a <table primary> in the FROM clause of a query.
    
    This new construct provides significant simplicity and performance
    benefit for XML data processing; what in a client-side custom
    implementation was reported to take 20 minutes can be executed in 400ms
    using XMLTABLE.  (The same functionality was said to take 10 seconds
    using nested PostgreSQL XPath function calls, and 5 seconds using
    XMLReader under PL/Python).
    
    The implemented syntax deviates slightly from what the standard
    requires.  First, the standard indicates that the PASSING clause is
    optional and that multiple XML input documents may be given to it; we
    make it mandatory and accept a single document only.  Second, we don't
    currently support a default namespace to be specified.
    
    This implementation relies on a new executor node based on a hardcoded
    method table.  (Because the grammar is fixed, there is no extensibility
    in the current approach; further constructs can be implemented on top of
    this such as JSON_TABLE, but they require changes to core code.)
    
    Author: Pavel Stehule, Álvaro Herrera
    Extensively reviewed by: Craig Ringer
    Discussion: https://postgr.es/m/CAFj8pRAgfzMD-LoSmnMGybD0WsEznLHWap8DO79+-GTRAPR4qA@mail.gmail.com
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