dbDelta( string|array $queries = '', bool $execute = true )

function dbDelta( $queries = '', $execute = true ) {
	global $wpdb;

	if ( in_array( $queries, array( '', 'all', 'blog', 'global', 'ms_global' ), true ) )
	    $queries = wp_get_db_schema( $queries );

	// Separate individual queries into an array
	if ( !is_array($queries) ) {
		$queries = explode( ';', $queries );
		$queries = array_filter( $queries );
	}

	/**
	 * Filters the dbDelta SQL queries.
	 *
	 * @since WP-3.3.0
	 *
	 * @param array $queries An array of dbDelta SQL queries.
	 */
	$queries = apply_filters( 'dbdelta_queries', $queries );

	$cqueries = array(); // Creation Queries
	$iqueries = array(); // Insertion Queries
	$for_update = array();

	// Create a tablename index for an array ($cqueries) of queries
	foreach ($queries as $qry) {
		if ( preg_match( "|CREATE TABLE ([^ ]*)|", $qry, $matches ) ) {
			$cqueries[ trim( $matches[1], '`' ) ] = $qry;
			$for_update[$matches[1]] = 'Created table '.$matches[1];
		} elseif ( preg_match( "|CREATE DATABASE ([^ ]*)|", $qry, $matches ) ) {
			array_unshift( $cqueries, $qry );
		} elseif ( preg_match( "|INSERT INTO ([^ ]*)|", $qry, $matches ) ) {
			$iqueries[] = $qry;
		} elseif ( preg_match( "|UPDATE ([^ ]*)|", $qry, $matches ) ) {
			$iqueries[] = $qry;
		} else {
			// Unrecognized query type
		}
	}

	/**
	 * Filters the dbDelta SQL queries for creating tables and/or databases.
	 *
	 * Queries filterable via this hook contain "CREATE TABLE" or "CREATE DATABASE".
	 *
	 * @since WP-3.3.0
	 *
	 * @param array $cqueries An array of dbDelta create SQL queries.
	 */
	$cqueries = apply_filters( 'dbdelta_create_queries', $cqueries );

	/**
	 * Filters the dbDelta SQL queries for inserting or updating.
	 *
	 * Queries filterable via this hook contain "INSERT INTO" or "UPDATE".
	 *
	 * @since WP-3.3.0
	 *
	 * @param array $iqueries An array of dbDelta insert or update SQL queries.
	 */
	$iqueries = apply_filters( 'dbdelta_insert_queries', $iqueries );

	$text_fields = array( 'tinytext', 'text', 'mediumtext', 'longtext' );
	$blob_fields = array( 'tinyblob', 'blob', 'mediumblob', 'longblob' );

	$global_tables = $wpdb->tables( 'global' );
	foreach ( $cqueries as $table => $qry ) {
		// Upgrade global tables only for the main site. Don't upgrade at all if conditions are not optimal.
		if ( in_array( $table, $global_tables ) && ! wp_should_upgrade_global_tables() ) {
			unset( $cqueries[ $table ], $for_update[ $table ] );
			continue;
		}

		// Fetch the table column structure from the database
		$suppress = $wpdb->suppress_errors();
		$tablefields = $wpdb->get_results("DESCRIBE {$table};");
		$wpdb->suppress_errors( $suppress );

		if ( ! $tablefields )
			continue;

		// Clear the field and index arrays.
		$cfields = $indices = $indices_without_subparts = array();

		// Get all of the field names in the query from between the parentheses.
		preg_match("|\((.*)\)|ms", $qry, $match2);
		$qryline = trim($match2[1]);

		// Separate field lines into an array.
		$flds = explode("\n", $qryline);

		// For every field line specified in the query.
		foreach ( $flds as $fld ) {
			$fld = trim( $fld, " \t\n\r\0\x0B," ); // Default trim characters, plus ','.

			// Extract the field name.
			preg_match( '|^([^ ]*)|', $fld, $fvals );
			$fieldname = trim( $fvals[1], '`' );
			$fieldname_lowercased = strtolower( $fieldname );

			// Verify the found field name.
			$validfield = true;
			switch ( $fieldname_lowercased ) {
				case '':
				case 'primary':
				case 'index':
				case 'fulltext':
				case 'unique':
				case 'key':
				case 'spatial':
					$validfield = false;

					/*
					 * Normalize the index definition.
					 *
					 * This is done so the definition can be compared against the result of a
					 * `SHOW INDEX FROM $table_name` query which returns the current table
					 * index information.
					 */

					// Extract type, name and columns from the definition.
					preg_match(
						  '/^'
						.   '(?P<index_type>'             // 1) Type of the index.
						.       'PRIMARY\s+KEY|(?:UNIQUE|FULLTEXT|SPATIAL)\s+(?:KEY|INDEX)|KEY|INDEX'
						.   ')'
						.   '\s+'                         // Followed by at least one white space character.
						.   '(?:'                         // Name of the index. Optional if type is PRIMARY KEY.
						.       '`?'                      // Name can be escaped with a backtick.
						.           '(?P<index_name>'     // 2) Name of the index.
						.               '(?:[0-9a-zA-Z$_-]|[\xC2-\xDF][\x80-\xBF])+'
						.           ')'
						.       '`?'                      // Name can be escaped with a backtick.
						.       '\s+'                     // Followed by at least one white space character.
						.   ')*'
						.   '\('                          // Opening bracket for the columns.
						.       '(?P<index_columns>'
						.           '.+?'                 // 3) Column names, index prefixes, and orders.
						.       ')'
						.   '\)'                          // Closing bracket for the columns.
						. '$/im',
						$fld,
						$index_matches
					);

					// Uppercase the index type and normalize space characters.
					$index_type = strtoupper( preg_replace( '/\s+/', ' ', trim( $index_matches['index_type'] ) ) );

					// 'INDEX' is a synonym for 'KEY', standardize on 'KEY'.
					$index_type = str_replace( 'INDEX', 'KEY', $index_type );

					// Escape the index name with backticks. An index for a primary key has no name.
					$index_name = ( 'PRIMARY KEY' === $index_type ) ? '' : '`' . strtolower( $index_matches['index_name'] ) . '`';

					// Parse the columns. Multiple columns are separated by a comma.
					$index_columns = $index_columns_without_subparts = array_map( 'trim', explode( ',', $index_matches['index_columns'] ) );

					// Normalize columns.
					foreach ( $index_columns as $id => &$index_column ) {
						// Extract column name and number of indexed characters (sub_part).
						preg_match(
							  '/'
							.   '`?'                      // Name can be escaped with a backtick.
							.       '(?P<column_name>'    // 1) Name of the column.
							.           '(?:[0-9a-zA-Z$_-]|[\xC2-\xDF][\x80-\xBF])+'
							.       ')'
							.   '`?'                      // Name can be escaped with a backtick.
							.   '(?:'                     // Optional sub part.
							.       '\s*'                 // Optional white space character between name and opening bracket.
							.       '\('                  // Opening bracket for the sub part.
							.           '\s*'             // Optional white space character after opening bracket.
							.           '(?P<sub_part>'
							.               '\d+'         // 2) Number of indexed characters.
							.           ')'
							.           '\s*'             // Optional white space character before closing bracket.
							.        '\)'                 // Closing bracket for the sub part.
							.   ')?'
							. '/',
							$index_column,
							$index_column_matches
						);

						// Escape the column name with backticks.
						$index_column = '`' . $index_column_matches['column_name'] . '`';

						// We don't need to add the subpart to $index_columns_without_subparts
						$index_columns_without_subparts[ $id ] = $index_column;

						// Append the optional sup part with the number of indexed characters.
						if ( isset( $index_column_matches['sub_part'] ) ) {
							$index_column .= '(' . $index_column_matches['sub_part'] . ')';
						}
					}

					// Build the normalized index definition and add it to the list of indices.
					$indices[] = "{$index_type} {$index_name} (" . implode( ',', $index_columns ) . ")";
					$indices_without_subparts[] = "{$index_type} {$index_name} (" . implode( ',', $index_columns_without_subparts ) . ")";

					// Destroy no longer needed variables.
					unset( $index_column, $index_column_matches, $index_matches, $index_type, $index_name, $index_columns, $index_columns_without_subparts );

					break;
			}

			// If it's a valid field, add it to the field array.
			if ( $validfield ) {
				$cfields[ $fieldname_lowercased ] = $fld;
			}
		}

		// For every field in the table.
		foreach ( $tablefields as $tablefield ) {
			$tablefield_field_lowercased = strtolower( $tablefield->Field );
			$tablefield_type_lowercased = strtolower( $tablefield->Type );

			// If the table field exists in the field array ...
			if ( array_key_exists( $tablefield_field_lowercased, $cfields ) ) {

				// Get the field type from the query.
				preg_match( '|`?' . $tablefield->Field . '`? ([^ ]*( unsigned)?)|i', $cfields[ $tablefield_field_lowercased ], $matches );
				$fieldtype = $matches[1];
				$fieldtype_lowercased = strtolower( $fieldtype );

				// Is actual field type different from the field type in query?
				if ($tablefield->Type != $fieldtype) {
					$do_change = true;
					if ( in_array( $fieldtype_lowercased, $text_fields ) && in_array( $tablefield_type_lowercased, $text_fields ) ) {
						if ( array_search( $fieldtype_lowercased, $text_fields ) < array_search( $tablefield_type_lowercased, $text_fields ) ) {
							$do_change = false;
						}
					}

					if ( in_array( $fieldtype_lowercased, $blob_fields ) && in_array( $tablefield_type_lowercased, $blob_fields ) ) {
						if ( array_search( $fieldtype_lowercased, $blob_fields ) < array_search( $tablefield_type_lowercased, $blob_fields ) ) {
							$do_change = false;
						}
					}

					if ( $do_change ) {
						// Add a query to change the column type.
						$cqueries[] = "ALTER TABLE {$table} CHANGE COLUMN `{$tablefield->Field}` " . $cfields[ $tablefield_field_lowercased ];
						$for_update[$table.'.'.$tablefield->Field] = "Changed type of {$table}.{$tablefield->Field} from {$tablefield->Type} to {$fieldtype}";
					}
				}

				// Get the default value from the array.
				if ( preg_match( "| DEFAULT '(.*?)'|i", $cfields[ $tablefield_field_lowercased ], $matches ) ) {
					$default_value = $matches[1];
					if ($tablefield->Default != $default_value) {
						// Add a query to change the column's default value
						$cqueries[] = "ALTER TABLE {$table} ALTER COLUMN `{$tablefield->Field}` SET DEFAULT '{$default_value}'";
						$for_update[$table.'.'.$tablefield->Field] = "Changed default value of {$table}.{$tablefield->Field} from {$tablefield->Default} to {$default_value}";
					}
				}

				// Remove the field from the array (so it's not added).
				unset( $cfields[ $tablefield_field_lowercased ] );
			} else {
				// This field exists in the table, but not in the creation queries?
			}
		}

		// For every remaining field specified for the table.
		foreach ($cfields as $fieldname => $fielddef) {
			// Push a query line into $cqueries that adds the field to that table.
			$cqueries[] = "ALTER TABLE {$table} ADD COLUMN $fielddef";
			$for_update[$table.'.'.$fieldname] = 'Added column '.$table.'.'.$fieldname;
		}

		// Index stuff goes here. Fetch the table index structure from the database.
		$tableindices = $wpdb->get_results("SHOW INDEX FROM {$table};");

		if ($tableindices) {
			// Clear the index array.
			$index_ary = array();

			// For every index in the table.
			foreach ($tableindices as $tableindex) {

				// Add the index to the index data array.
				$keyname = strtolower( $tableindex->Key_name );
				$index_ary[$keyname]['columns'][] = array('fieldname' => $tableindex->Column_name, 'subpart' => $tableindex->Sub_part);
				$index_ary[$keyname]['unique'] = ($tableindex->Non_unique == 0)?true:false;
				$index_ary[$keyname]['index_type'] = $tableindex->Index_type;
			}

			// For each actual index in the index array.
			foreach ($index_ary as $index_name => $index_data) {

				// Build a create string to compare to the query.
				$index_string = '';
				if ($index_name == 'primary') {
					$index_string .= 'PRIMARY ';
				} elseif ( $index_data['unique'] ) {
					$index_string .= 'UNIQUE ';
				}
				if ( 'FULLTEXT' === strtoupper( $index_data['index_type'] ) ) {
					$index_string .= 'FULLTEXT ';
				}
				if ( 'SPATIAL' === strtoupper( $index_data['index_type'] ) ) {
					$index_string .= 'SPATIAL ';
				}
				$index_string .= 'KEY ';
				if ( 'primary' !== $index_name  ) {
					$index_string .= '`' . $index_name . '`';
				}
				$index_columns = '';

				// For each column in the index.
				foreach ($index_data['columns'] as $column_data) {
					if ( $index_columns != '' ) {
						$index_columns .= ',';
					}

					// Add the field to the column list string.
					$index_columns .= '`' . $column_data['fieldname'] . '`';
				}

				// Add the column list to the index create string.
				$index_string .= " ($index_columns)";

				// Check if the index definition exists, ignoring subparts.
				if ( ! ( ( $aindex = array_search( $index_string, $indices_without_subparts ) ) === false ) ) {
					// If the index already exists (even with different subparts), we don't need to create it.
					unset( $indices_without_subparts[ $aindex ] );
					unset( $indices[ $aindex ] );
				}
			}
		}

		// For every remaining index specified for the table.
		foreach ( (array) $indices as $index ) {
			// Push a query line into $cqueries that adds the index to that table.
			$cqueries[] = "ALTER TABLE {$table} ADD $index";
			$for_update[] = 'Added index ' . $table . ' ' . $index;
		}

		// Remove the original table creation query from processing.
		unset( $cqueries[ $table ], $for_update[ $table ] );
	}

	$allqueries = array_merge($cqueries, $iqueries);
	if ($execute) {
		foreach ($allqueries as $query) {
			$wpdb->query($query);
		}
	}

	return $for_update;
}