WL#8481: Callback for computation of virtual column index values from InnoDB purge threads
Affects: Server-Prototype Only — Status: Complete
InnoDB needs to be able to compute virtual column index values to implement WL#8149 B-tree Index Support on non-materialized virtual columns Generally this is done from connection threads. This is handled by WL#8227 Support SEs to create index on virtual generated columns However it also needs to happen from InnoDB purge threads. These threads do not correspond to connections, and thus don't have THDs or access to TABLE objects. This WL is about providing a server layer functions that can be called by purge threads. These functions will provide: - A way to construct and destroy THD objects - A way to do virtual column index value computation when no TABLE object is available. This includes taking a metadata lock on the table, opening the table, doing computation (see WL#8227), releasing lock and closing table. These server layer functions will not be a part of a public API, but only designed from use from InnoDB. Opening and closing the table for every call can be costly, but prototyping of WL#8149 shows that this processing will be done very rarely. Then it is advantageous to do it the way described above as it minimizes lock duration.
F-1: The worklog will provide a function for creating THD objects. This is at first intended for use from InnoDB purge threads, but should be able to be used elsewhere as well. A single API call should create THD and initialize it - this includes initialization of thread local storage and. performance schema instrumentation. F-2: Purge threads should be able to create THDs when the purge threads are created. This means during server startup. At this point, the server has not been initialized enough to create THDs with support for loading dynamic plugins. This means it must be possible to create THDs with this support disabled. F-3: The worklog will provide a function for cleaning up and destroying THD objects. F-4: The worklog will provide a function to allow InnoDB to build a row template for tables with generated columns. The function must be callable from purge threads. This function must: a) Take a metadata lock on the table b) Open the table without calling ha_open() c) Call a function provided by InnoDB with the TABLE opened d) Close the table and release the metadata lock. F-5: The worklog will provide a function to allow InnoDB to evaluate generated columns. The function must be callable from purge threads. This function must: a) Take a metadata lock on the table b) Open the table. This must optionally (specified by caller) call ha_open(). c) Call function provided by WL#8227 to do the computation. d) Close the table and release the metadata lock.
1) Creation of THDs =================== /** Create a THD and do proper initialization of it. @param enable_plugins Should dynamic plugin support be enabled? @param background_thread Is this a background thread? @note Dynamic plugin support is only possible for THDs that are created after the server has initialized properly. @note THDs for background threads are currently not added to the global THD list. So they will e.g. not be visible in SHOW PROCESSLIST and the server will not wait for them to terminate during shutdown. */ THD *create_thd(bool enable_plugins, bool background_thread); 2) Destroying THDs ================== /** Cleanup the THD object, remove it from the global list of THDs and delete it. @param THD pointer to THD object. */ void destroy_thd(THD *thd); 3) Creating generated column templates ====================================== /** Callback to allow InnoDB to prepare a template for generated column processing. This function will open the table without opening in the engine and call the provided function with the TABLE object made. The function will then close the TABLE. @param thd Thread handle @param db_name Name of database containing the table @param table_name Name of table to open @param myc InnoDB function to call for processing TABLE @param ib_table Argument for InnoDB function @return true in case of error, false otherwise. */ typedef void (*ib_v_templ_callback_t)(const TABLE*, void*); bool my_prepare_gcolumn_template(THD *thd, const char *db_name, const char *table_name, ib_v_templ_callback_t myc, void* ib_table); 4) Computing generated column values ==================================== /** Callback for generated columns processing. Will open the table and call handler::my_eval_gcolumn_expr() to do the actual processing. This function is intended for use when no TABLE object already exists - e.g. from purge threads. @param thd Thread handle @param open_in_engine Should ha_open() be called? @param db_name Database containing the table to open @param table_name Name of table to open @param fields Bitmap of field index of evaluated generated column @param record Record buffer @return true in case of error, false otherwise. */ bool my_eval_gcolumn_expr(THD *thd, bool open_in_engine, const char *db_name, const char *table_name, ulonglong fields, char *record); Note that as part of 1+2, thread pool THD initialization and destroy can be refactored to simply implementation and reuse API provided here.
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