History: InnoDB

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            ! About InnoDB
InnoDB is a [https://en.wikipedia.org/wiki/Database_engine|storage engine] for the databases supported by Tiki: ((MariaDB)), ((MySQL)) and ((Percona Server for MySQL)).

Support for InnoDB was introduced in ((Tiki8)) (2011). InnoDB is the default storage engine since ((Tiki18)) (2017) and MariaDB 10.2 (2017)

InnoDB provides several benefits over the MyISAM engine. It is crash-proof (the database will automatically recover errors upon restart).

Additionally, InnoDB supports (multi-statement) transactions and row-level locking, i.e. updates, inserts and deletes will no longer lock the whole file/table. It also supports referential integrity by the use of foreign keys. But these features are currently not used by vanilla Tiki. Customized Tiki versions could take advantage of these features, e.g. if the tiki_pages table is linked from a custom table.

The source code lives here: https://gitlab.com/tikiwiki/tiki/-/blob/master/db/tiki_convert_myisam_to_innodb.sql

! Installing with InnoDB
When creating the Tiki database, the user can choose to use MyISAM or InnoDB. Once the choice is made, Tiki will remember the selection.

! Migrating existing MyISAM databases
Migration steps:
# In your Tiki instance, look for the file: db/tiki_convert_myisam_to_innodb.sql
# Make sure you are not using the deprecated tiki-searchresults.php feature, but instead use tiki-searchindex.php
# Do a full backup of your current database/installation
# Alter the database engine for all tables to InnoDB using db/tiki_convert_myisam_to_innodb.sql

Performing the conversion with phpMyAdmin may cause a PHP timeout. Running the script with the mysql command avoids that:
{CODE(colors="shell" theme="default")}mysql -u userName -p databaseName < db\tiki_convert_myisam_to_innodb.sql{CODE}
Nevertheless, step 4 may fail converting the tiki_files table if innodb_log_file_size is not high enough. In one case, 25 MB was insufficient (80 MB sufficed).


! Supporting InnoDB
!! Installs
Rules
* No FKs are allowed in the tiki.sql script.
* Fulltext index definitions are placed in the tiki_myisam.sql file. InnoDB specific definitions are placed in the tiki_innodb.sql file.
* The word MyISAM must be used (only) in the engine specification, and it is not allowed in attribute names or other definitions.

The Tiki installer translates the engine type, based on the selected database in the installer GUI.

After the main install script (tiki.sql), the installer will run tiki_myisam.sql or tiki_innodb.sql for the respective installation. tiki_myisam.sql installs the fulltext indexes.

!! Upgrades
When adding an engine-dependent patch, the engine-independent SQL statements must be put in the YYYYMMDD_description_tiki.sql file. The engine-dependent parts must be put in an accompanying PHP file, defining a post_YYYYMMDD_description_tiki function.
Example: Add a table with a fulltext index
{CODE(caption="20110918_tiki_test_tiki.sql" wrap="0")}DROP TABLE IF EXISTS `tiki_test`;
CREATE TABLE `tiki_test` (
  `title` varchar(255) default NULL,
  KEY `title` (`title`)
) ENGINE=MyISAM;{CODE}
and a PHP file specifying the engine dependent part
{CODE(caption="20110918_tiki_test_tiki.php")}function post_20110918_tiki_test_tiki( $installer ) {
  if($installer->isMySQLFulltextSearchSupported()) {
    $installer->query( "CREATE FULLTEXT INDEX ft_test ON tiki_test(`title`);");
  }
}
{CODE}

!! Continuous Integration
The [https://gitlab.com/tikiwiki/tiki/-/blob/master/.gitlab-ci.yml|Tiki CI] has sql-engine-conversion section which leverages doc/devtools/check_sql_engine_conversion.php 

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See also: [https://dev.tiki.org/ExperimentalBranches+InnoDB|ExperimentalBranches InnoDB]