diff --git a/cli/class-tmdo-cli.php b/cli/class-tmdo-cli.php
index d87c79c..b44bd5b 100644
--- a/cli/class-tmdo-cli.php
+++ b/cli/class-tmdo-cli.php
@@ -142,6 +142,10 @@ class TMDO_CLI {
 		foreach ( $violations as $entity_type => $integrations ) {
 			WP_CLI::warning( "  [GUARD] {$entity_type} is aeav_only while " . implode( ', ', $integrations ) . " reads native meta — write flat back with `wp tmdo bridge-restore-native {$entity_type} --demote`." );
 		}
+		$pending = TMDO_Schema_Manager::pending_indexes();
+		if ( $pending ) {
+			WP_CLI::warning( '  [INDEX] declared indexes not added yet: ' . implode( ', ', $pending ) . ' — still missing after this run: another process is adding them, or the ALTER failed (see the `schema.index_failed` log).' );
+		}
 
 		$status = get_option( TMDO_Drift_Check::OPTION, array() );
 		$status = is_array( $status ) ? $status : array();
diff --git a/includes/class-tmdo-core.php b/includes/class-tmdo-core.php
index fa8fb2e..3a116bb 100644
--- a/includes/class-tmdo-core.php
+++ b/includes/class-tmdo-core.php
@@ -306,6 +306,9 @@ class TMDO_Core {
 		add_action( 'add_option_' . TMDO_Hook_Bus_Bridge::OPTION, array( 'TMDO_Drift_Check', 'on_hook_bus_added' ), 10, 2 );
 		add_action( TMDO_Drift_Check::RESYNC_HOOK, array( 'TMDO_Drift_Check', 'run_scheduled_fix' ) );
 
+		// 1.3.0: declared indexes a visitor request left for WP-Cron.
+		add_action( TMDO_Schema_Manager::INDEXES_HOOK, array( 'TMDO_Schema_Manager', 'ensure_pending_indexes' ) );
+
 		// v2.3.0 M6: daily snapshot prune cron (paired with health check).
 		add_action( 'wpdo_snapshot_prune_daily', array( 'TMDO_Snapshot_Pruner', 'cron_run' ) );
 
diff --git a/includes/class-tmdo-installer.php b/includes/class-tmdo-installer.php
index 7dd5fe8..da1244a 100644
--- a/includes/class-tmdo-installer.php
+++ b/includes/class-tmdo-installer.php
@@ -789,6 +789,7 @@ class TMDO_Installer {
 		wp_clear_scheduled_hook( 'wpdo_post_shadow_verify' );
 		wp_clear_scheduled_hook( 'wpdo_term_comment_shadow_verify' );
 		wp_clear_scheduled_hook( 'tmdo_drift_resync' );
+		wp_clear_scheduled_hook( 'tmdo_ensure_group_indexes' );
 		wp_clear_scheduled_hook( 'wpdo_post_stress_test_batch' );
 		wp_clear_scheduled_hook( 'wpdo_user_stress_test_batch' );
 		wp_clear_scheduled_hook( 'wpdo_term_stress_test_batch' );
@@ -993,6 +994,7 @@ class TMDO_Installer {
 			'wpdo_post_shadow_verify',
 			'wpdo_term_comment_shadow_verify',
 			'tmdo_drift_resync',
+			'tmdo_ensure_group_indexes',
 			'wpdo_post_stress_test_batch',
 			'wpdo_user_stress_test_batch',
 			'wpdo_term_stress_test_batch',
diff --git a/includes/engine/class-tmdo-schema-manager.php b/includes/engine/class-tmdo-schema-manager.php
index 448c21e..88ee68b 100644
--- a/includes/engine/class-tmdo-schema-manager.php
+++ b/includes/engine/class-tmdo-schema-manager.php
@@ -23,6 +23,9 @@ final class TMDO_Schema_Manager {
 	/** Option: "entity/group" → md5 of the composite index definition already applied. */
 	public const INDEXES_OPTION = 'tmdo_entity_indexes';
 
+	/** Single cron event that adds declared indexes a visitor request had to leave out. */
+	public const INDEXES_HOOK = 'tmdo_ensure_group_indexes';
+
 	/** Request-scoped cache for table_exists() — prevents repeated SHOW TABLES per request. */
 	private static array $table_exists_cache = array();
 
@@ -144,6 +147,11 @@ final class TMDO_Schema_Manager {
 			}
 		}
 
+		// A new table gets its declared composite indexes while it is empty.
+		foreach ( TMDO_Entity_Registry::get_group_indexes( $entity_type, $group_name ) as $name => $keys ) {
+			$sql_indexes[] = 'KEY `cidx_' . self::sanitize_column_name( (string) $name ) . '` (' . implode( ', ', array_map( static fn( $key ) => '`' . self::sanitize_column_name( $key ) . '`', $keys ) ) . ')';
+		}
+
 		$columns_sql = implode( ",\n  ", $sql_columns );
 		$indexes_sql = implode( ",\n  ", $sql_indexes );
 
@@ -176,6 +184,12 @@ final class TMDO_Schema_Manager {
 	 * applied definition is remembered per group (one autoloaded option), so
 	 * requests only run SHOW INDEX / ALTER when the declaration changes.
 	 *
+	 * On a large table the ALTER takes seconds, so it runs only from WP-CLI or
+	 * WP-Cron (ddl_allowed(); other requests schedule INDEXES_HOOK) and in one
+	 * process at a time (MySQL GET_LOCK on index_lock_name()). Until every
+	 * ALTER succeeds the group stays in pending_indexes() and the table works
+	 * without the index.
+	 *
 	 * @param string $entity_type Entity type.
 	 * @param string $group_name  Group name.
 	 * @return void
@@ -193,23 +207,112 @@ final class TMDO_Schema_Manager {
 		if ( ( $applied[ $id ] ?? '' ) === $signature ) {
 			return;
 		}
+		if ( ! self::ddl_allowed() ) {
+			if ( ! wp_next_scheduled( self::INDEXES_HOOK ) ) {
+				wp_schedule_single_event( time(), self::INDEXES_HOOK );
+			}
+			return;
+		}
 		$table = self::get_table_name( $entity_type, $group_name );
 		if ( ! self::table_exists( $table ) ) {
 			return;
 		}
 
-		$existing = array_unique( array_column( (array) $wpdb->get_results( "SHOW INDEX FROM `{$table}`", ARRAY_A ), 'Key_name' ) );
-		foreach ( $declared as $name => $keys ) {
-			$index = 'cidx_' . self::sanitize_column_name( $name );
-			if ( in_array( $index, $existing, true ) ) {
-				continue;
-			}
-			$columns = implode( ', ', array_map( static fn( $key ) => '`' . self::sanitize_column_name( $key ) . '`', $keys ) );
-			$wpdb->query( "ALTER TABLE `{$table}` ADD INDEX `{$index}` ({$columns})" );
+		// add_option() is INSERT ... ON DUPLICATE KEY UPDATE and would let two
+		// processes both "acquire" an option lock; GET_LOCK is atomic and is
+		// released by MySQL if this process dies.
+		$lock = self::index_lock_name();
+		if ( '1' !== (string) $wpdb->get_var( $wpdb->prepare( 'SELECT GET_LOCK(MD5(CONCAT(DATABASE(), %s)), 0)', $lock ) ) ) {
+			return; // Another process is adding indexes.
 		}
+		try {
+			// SHOW INDEX (not the option, cached for this request) tells what another process already added.
+			$existing = array_unique( array_column( (array) $wpdb->get_results( "SHOW INDEX FROM `{$table}`", ARRAY_A ), 'Key_name' ) );
+			$failed   = false;
+			foreach ( $declared as $name => $keys ) {
+				$index = 'cidx_' . self::sanitize_column_name( $name );
+				if ( in_array( $index, $existing, true ) ) {
+					continue;
+				}
+				$columns = implode( ', ', array_map( static fn( $key ) => '`' . self::sanitize_column_name( $key ) . '`', $keys ) );
+				if ( false === $wpdb->query( "ALTER TABLE `{$table}` ADD INDEX `{$index}` ({$columns})" ) ) {
+					$failed = true;
+					TMDO_Logger::warning(
+						'schema.index_failed',
+						array(
+							'table' => $table,
+							'index' => $index,
+							'error' => $wpdb->last_error,
+						)
+					);
+				}
+			}
 
-		$applied[ $id ] = $signature;
-		update_option( self::INDEXES_OPTION, $applied, true );
+			if ( ! $failed ) {
+				$applied[ $id ] = $signature;
+				update_option( self::INDEXES_OPTION, $applied, true );
+			}
+		} finally {
+			$wpdb->query( $wpdb->prepare( 'SELECT RELEASE_LOCK(MD5(CONCAT(DATABASE(), %s)))', $lock ) );
+		}
+	}
+
+	/**
+	 * INDEXES_HOOK handler: add the indexes a visitor request could not.
+	 *
+	 * @return void
+	 */
+	public static function ensure_pending_indexes(): void {
+		foreach ( self::pending_indexes() as $id ) {
+			list( $entity_type, $group_name ) = explode( '/', $id, 2 );
+			self::ensure_group_indexes( $entity_type, $group_name );
+		}
+	}
+
+	/**
+	 * Key of the MySQL lock held while indexes are added. Lock names are
+	 * server-wide, so SQL hashes it with DATABASE() to keep sites apart.
+	 *
+	 * @return string
+	 */
+	public static function index_lock_name(): string {
+		global $wpdb;
+		return $wpdb->prefix . self::INDEXES_OPTION;
+	}
+
+	/**
+	 * Whether this request may alter flat tables: WP-CLI or WP-Cron only.
+	 * Front-end, REST, admin-ajax, admin-post and even /wp-admin/ requests
+	 * (init runs before auth_redirect()) can be a visitor's, who would wait
+	 * for the ALTER. Filter: `tmdo_schema_ddl_allowed`.
+	 *
+	 * @return bool
+	 */
+	public static function ddl_allowed(): bool {
+		$allowed = ( defined( 'WP_CLI' ) && WP_CLI )
+			|| ( function_exists( 'wp_doing_cron' ) && wp_doing_cron() );
+		return (bool) apply_filters( 'tmdo_schema_ddl_allowed', $allowed );
+	}
+
+	/**
+	 * Registered groups whose declared indexes are not on their table yet.
+	 *
+	 * @return string[] "entity/group"
+	 */
+	public static function pending_indexes(): array {
+		$applied = get_option( self::INDEXES_OPTION, array() );
+		$applied = is_array( $applied ) ? $applied : array();
+		$pending = array();
+		foreach ( TMDO_Entity_Registry::VALID_ENTITY_TYPES as $entity_type ) {
+			foreach ( TMDO_Entity_Registry::get_groups_for_type( $entity_type ) as $group_name ) {
+				$declared = TMDO_Entity_Registry::get_group_indexes( $entity_type, $group_name );
+				if ( $declared && ( $applied[ "{$entity_type}/{$group_name}" ] ?? '' ) !== md5( (string) wp_json_encode( $declared ) )
+					&& self::table_exists( self::get_table_name( $entity_type, $group_name ) ) ) {
+					$pending[] = "{$entity_type}/{$group_name}";
+				}
+			}
+		}
+		return $pending;
 	}
 
 	/**
diff --git a/tests/integration/GroupCompositeIndexTest.php b/tests/integration/GroupCompositeIndexTest.php
index 4e4d4f5..e8d6e3e 100644
--- a/tests/integration/GroupCompositeIndexTest.php
+++ b/tests/integration/GroupCompositeIndexTest.php
@@ -31,6 +31,10 @@ class GroupCompositeIndexTest extends TestCase {
 		$wpdb->query( 'CREATE TABLE `' . self::TABLE . '` ( id BIGINT NOT NULL AUTO_INCREMENT, post_id BIGINT NOT NULL, ci_flag BIGINT(20) DEFAULT NULL, ci_price DECIMAL(18,6) DEFAULT NULL, PRIMARY KEY (id), UNIQUE KEY uk_entity (post_id) ) ENGINE=InnoDB' );
 		$wpdb->query( 'INSERT INTO `' . self::TABLE . '` (post_id, ci_flag, ci_price) VALUES (1, 1, 10), (2, 0, 20)' );
 		$GLOBALS['_wp_options'] = array();
+		// Index DDL runs from WP-CLI / WP-Cron only (2.3 PR-090a); these tests act as one.
+		$GLOBALS['_wp_filter_callbacks']['tmdo_schema_ddl_allowed'] = array();
+		add_filter( 'tmdo_schema_ddl_allowed', static fn() => true );
+		$GLOBALS['_wp_scheduled_single'] = array();
 
 		$ref = new ReflectionProperty( WPDO_Schema_Manager::class, 'table_exists_cache' );
 		$ref->setAccessible( true );
@@ -53,6 +57,81 @@ class GroupCompositeIndexTest extends TestCase {
 		global $wpdb;
 		$wpdb->query( 'DROP TABLE IF EXISTS `' . self::TABLE . '`' );
 		WPDO_Entity_Registry::init();
+		$GLOBALS['_wp_filter_callbacks']['tmdo_schema_ddl_allowed'] = array();
+		unset( $GLOBALS['_wp_is_admin'], $GLOBALS['_wp_scheduled_single'] );
+	}
+
+	public function test_a_web_request_adds_no_index_and_leaves_it_to_wp_cron(): void {
+		$GLOBALS['_wp_filter_callbacks']['tmdo_schema_ddl_allowed'] = array();
+		foreach ( array( false, true ) as $is_admin ) { // /wp-admin/ runs init before auth_redirect().
+			$GLOBALS['_wp_is_admin'] = $is_admin;
+			$this->assertFalse( TMDO_Schema_Manager::ddl_allowed() );
+
+			TMDO_Schema_Manager::ensure_group_indexes( 'post', 'ci_core' );
+
+			$this->assertArrayNotHasKey( 'cidx_flag_price', $this->indexes(), 'no ALTER in a web request' );
+		}
+		$this->assertSame( array( 'post/ci_core' ), TMDO_Schema_Manager::pending_indexes() );
+		$this->assertContains( TMDO_Schema_Manager::INDEXES_HOOK, $GLOBALS['_wp_scheduled_single'] );
+
+		add_filter( 'tmdo_schema_ddl_allowed', static fn() => true ); // The cron run.
+		TMDO_Schema_Manager::ensure_pending_indexes();
+
+		$this->assertArrayHasKey( 'cidx_flag_price', $this->indexes() );
+		$this->assertSame( array(), TMDO_Schema_Manager::pending_indexes() );
+	}
+
+	public function test_the_ddl_decision_can_be_overridden(): void {
+		add_filter( 'tmdo_schema_ddl_allowed', static fn() => false );
+
+		TMDO_Schema_Manager::ensure_group_indexes( 'post', 'ci_core' );
+
+		$this->assertArrayNotHasKey( 'cidx_flag_price', $this->indexes() );
+	}
+
+	public function test_a_failed_alter_is_not_recorded_as_applied(): void {
+		global $wpdb;
+		$wpdb->query( 'ALTER TABLE `' . self::TABLE . '` ADD COLUMN ci_note TEXT DEFAULT NULL' );
+		WPDO_Entity_Registry::init();
+		WPDO_Entity_Registry::register_adapter( 'post', new WPDO_Adapter_Post() );
+		WPDO_Entity_Registry::register_group(
+			'post',
+			'ci_core',
+			array(
+				array( 'key' => 'ci_flag', 'type' => 'integer' ),
+				array( 'key' => 'ci_note', 'type' => 'textarea' ),
+			),
+			array( 'indexes' => array( 'flag_note' => array( 'ci_flag', 'ci_note' ) ) ) // TEXT without a prefix length: MySQL 1170.
+		);
+
+		TMDO_Schema_Manager::ensure_group_indexes( 'post', 'ci_core' );
+
+		$this->assertArrayNotHasKey( 'cidx_flag_note', $this->indexes() );
+		$this->assertFalse( get_option( TMDO_Schema_Manager::INDEXES_OPTION ), 'not recorded, so it is retried' );
+		$this->assertSame( array( 'post/ci_core' ), TMDO_Schema_Manager::pending_indexes() );
+	}
+
+	public function test_a_lock_held_by_another_process_skips_the_ddl_until_it_is_released(): void {
+		$other = new mysqli(
+			getenv( 'TMDO_TEST_DB_HOST' ) ?: '127.0.0.1',
+			getenv( 'TMDO_TEST_DB_USER' ) ?: 'dbo',
+			(string) getenv( 'TMDO_TEST_DB_PASS' ),
+			getenv( 'TMDO_TEST_DB_NAME' ) ?: 'wp_wpdo_test'
+		);
+		$name = $other->real_escape_string( TMDO_Schema_Manager::index_lock_name() );
+		$this->assertSame( '1', (string) $other->query( "SELECT GET_LOCK(MD5(CONCAT(DATABASE(), '{$name}')), 0)" )->fetch_row()[0] );
+
+		TMDO_Schema_Manager::ensure_group_indexes( 'post', 'ci_core' );
+		$this->assertArrayNotHasKey( 'cidx_flag_price', $this->indexes(), 'another process holds the lock' );
+
+		$other->query( "SELECT RELEASE_LOCK(MD5(CONCAT(DATABASE(), '{$name}')))" );
+		$other->close();
+		TMDO_Schema_Manager::ensure_group_indexes( 'post', 'ci_core' );
+
+		$this->assertArrayHasKey( 'cidx_flag_price', $this->indexes() );
+		$this->assertSame( array(), TMDO_Schema_Manager::pending_indexes() );
+		global $wpdb;
+		$this->assertSame( '1', (string) $wpdb->get_var( $wpdb->prepare( 'SELECT IS_FREE_LOCK(MD5(CONCAT(DATABASE(), %s)))', TMDO_Schema_Manager::index_lock_name() ) ), 'lock released' );
 	}
 
 	/** @return array<string, string[]> index name → ordered columns */
diff --git a/tests/integration/bootstrap.php b/tests/integration/bootstrap.php
index 6d3dad8..1c9cbcc 100644
--- a/tests/integration/bootstrap.php
+++ b/tests/integration/bootstrap.php
@@ -168,7 +168,12 @@ $wpdb = new class( $_mysqli ) {
 			fn( $v ) => $v === null ? 'NULL' : "'" . $this->db->real_escape_string( (string) $v ) . "'",
 			array_values( $data )
 		) );
-		$ok = $this->db->query( "INSERT INTO `{$table}` ({$cols}) VALUES ({$vals})" );
+		try {
+			$ok = $this->db->query( "INSERT INTO `{$table}` ({$cols}) VALUES ({$vals})" );
+		} catch ( mysqli_sql_exception $e ) { // Like wpdb::insert(): an SQL error returns false.
+			$this->last_error = $e->getMessage();
+			return false;
+		}
 		if ( $ok ) { $this->insert_id = (int) $this->db->insert_id; return $this->insert_id; }
 		$this->last_error = (string) $this->db->error;
 		return false;
@@ -200,7 +205,12 @@ $wpdb = new class( $_mysqli ) {
 	public function flush(): void {}
 
 	public function query( string $sql ): int|bool {
-		$result = $this->db->query( $sql );
+		try {
+			$result = $this->db->query( $sql );
+		} catch ( mysqli_sql_exception $e ) { // Like wpdb::query(): an SQL error returns false.
+			$this->last_error = $e->getMessage();
+			return false;
+		}
 		if ( $result instanceof mysqli_result ) { $result->free(); return true; }
 		if ( false === $result ) { return false; }
 		return $this->db->affected_rows;
@@ -291,7 +301,7 @@ if ( ! function_exists( 'is_singular' ) )    { function is_singular( $t = '' ):
 if ( ! function_exists( 'wp_doing_ajax' ) )  { function wp_doing_ajax(): bool { return false; } }
 if ( ! function_exists( 'wp_next_scheduled' ) ) { function wp_next_scheduled( string $hook ): int|false { return false; } }
 if ( ! function_exists( 'wp_schedule_event' ) ) { function wp_schedule_event( int $t, string $r, string $h ): bool { return true; } }
-if ( ! function_exists( 'wp_schedule_single_event' ) ) { function wp_schedule_single_event( int $ts, string $hook ): bool { return true; } }
+if ( ! function_exists( 'wp_schedule_single_event' ) ) { function wp_schedule_single_event( int $ts, string $hook ): bool { $GLOBALS['_wp_scheduled_single'][] = $hook; return true; } }
 if ( ! function_exists( 'wp_clear_scheduled_hook' ) ) { function wp_clear_scheduled_hook( string $hook ): int|false { return 0; } }
 if ( ! function_exists( 'sanitize_text_field' ) ) { function sanitize_text_field( string $s ): string { return trim( strip_tags( $s ) ); } }
 if ( ! function_exists( '__' ) ) { function __( string $text, string $domain = 'default' ): string { return $text; } }