# RFC 89: SQL logging callback

Author: | Alessandro Pasotti
Contact: | elpaso @ itopen.it
Started: | 2022-Nov-30
Status: | Adopted, implemented
Target: | GDAL 3.7

## Summary

The document proposes and describes the introduction of a new callback function to allow applications that use the GDAL library to monitor the SQL queries actually sent from GDAL to the database.

## Motivation

Applications (for example QGIS) may provide a debugging panel that allow users to monitor the SQL command that are sent to the layers stored on databases.

This panel implemented in QGIS is very useful to identify erroneous or inefficient queries and also made the users more conscious about what is going on in the application when layers are loaded and features are fetched.

## Technical details

The implementation details are still to be defined, a possible implementation would provide a method to GDALDataset that allows client code to set the callback function (with some opaque context to be passed back to client code).

The callback function will then be called from the drivers that make database calls each time a SQL command is sent to the backend, in case of prepared queries the actual SQL after parameter substitutions should be sent.

The callback will provide additional information about the executed query (if available):

- error string message

- number of affected/retrieved records

- time taken to execute the query

Example API:

    // Function signature
    typedef void (*GDALQueryLoggerFunc)(const char *pszSQL, const char *pszError, int64_t lNumRecords, int64_t lExecutionTimeMilliseconds, void *pQueryLoggerArg);

    bool CPL_DLL GDALDatasetSetQueryLoggerFunc(GDALDatasetH hDS, GDALQueryLoggerFunc pfnQueryLoggerFunc, void* poQueryLoggerArg );

    // C-API
    bool CPL_DLL GDALDatasetSetQueryLoggerFunc(GDALDatasetH hDS, GDALQueryLoggerFunc pfnQueryLoggerFunc, void* poQueryLoggerArg );
        GDALQueryLoggerFunc GDALDatasetQueryLoggerFunc( GDALDatasetH hDS );

    // Function call from the driver
    if ( m_poDS->pfQueryLoggerFunc )
    {
        // -1 for time and num records means no information available
        m_poDS->pfQueryLoggerFunc( soSQL.c_str(), nullptr, elapsedTime, numAffectedRecords, m_poDS->QueryLoggerArg() );
    }

The callback function will be initially used by the SQLite-based drivers only (GPKG included).

The callback may be called by multiple threads in a concurrent way, for this reason the implementation of the callback should be robust to that (use of lock typically). That problem may arise in drivers that use multi-threading as an optimization implementation detail. And this is typically the case of the GeoPackage driver in its Arrow stream interface.

## Efficiency considerations

Drivers that support the query logger callback function would need to check if the function pointer is nullptr and call the function if it is not. The cost of thish check is probably negligible on most architectures.

In order to catch SQLite prepare errors, a prepare function wrapper will be called instead of the sqlite3 API C function, this implies the cost of a function call (which might be inlined if necessary).

Comparison of current master release build with the proposed implementation on a 145665553 feature 15GB GPKG Point layer stored on fast SSD:

Benchmark | Master | Proposed implementation
bench_ogr_batch | 0.13m | 0.12m
bench_ogr_c_capi | 2.57m | 2.57m

## Backward compatibility

None.

The callback would be set from a new method, no changes to the class constructor would be needed.

## SWIG Bindings

This implementation will not be exposed to bindings.

## Testing

A C++ test will be added to the test suite.

## Voting history

+1 from PSC members MateuszL, JukkaR, TamasS and EvenR