The InnoDB adaptive hash index is sometimes degrading the performance of InnoDB, and it is sometimes disabled to get more consistent performance. We should have a compile-time option to disable the adaptive hash index.

Let us introduce two options:

OPTION(WITH_INNODB_AHI "Include innodb_adaptive_hash_index" ON)

OPTION(WITH_INNODB_ROOT_GUESS "Cache index root block descriptors" ON)

IF(WITH_INNODB_AHI)

  ADD_DEFINITIONS(-DBTR_CUR_HASH_ADAPT -DBTR_CUR_ADAPT)

  IF(NOT WITH_INNODB_ROOT_GUESS)

    MESSAGE(WARNING "WITH_INNODB_AHI implies WITH_INNODB_ROOT_GUESS")

  ENDIF()

ELSEIF(WITH_INNODB_ROOT_GUESS)

  ADD_DEFINITIONS(-DBTR_CUR_ADAPT)

ENDIF()

And let us consistently enclose all dependent code in appropriate #ifdef.

As part of this change, the misleadingly named function trx_search_latch_release_if_reserved(trx) will be replaced with the macro trx_assert_no_search_latch(trx) that will be empty unless BTR_CUR_HASH_ADAPT is defined (cmake -DWITH_INNODB_AHI=ON).

We will remove the unused column INFORMATION_SCHEMA.INNODB_TRX.TRX_ADAPTIVE_HASH_TIMEOUT.
In MariaDB Server 10.1, it used to reflect the value of trx_t::search_latch_timeout which could be adjusted during row_search_for_mysql(). In 10.2, there is no such field.

Apart from the removal of the unused column, this is an almost non-functional change to the server when using the default build options, because WITH_INNODB_AHI will be ON by default.