Persistent Connections
Notes
Mysqlnd
PHP Manual

# Statistics

Using Statistical Data

MySQL Native Driver contains support for gathering statistics on the communication between the client and the server. The statistics gathered are of two main types:

- Client statistics

- Connection statistics

If you are using the mysqli extension, these statistics can be obtained through two API calls:

- mysqli_get_client_stats()

- mysqli_get_connection_stats()

> Note:

> Statistics are aggregated among all extensions that use MySQL Native Driver. For example, when compiling both ext/mysql and ext/mysqli against MySQL Native Driver, both function calls of ext/mysql and ext/mysqli will change the statistics. There is no way to find out how much a certain API call of any extension that has been compiled against MySQL Native Driver has impacted a certain statistic. You can configure the PDO MySQL Driver, ext/mysql and ext/mysqli to optionally use the MySQL Native Driver. When doing so, all three extensions will change the statistics.

Accessing Client Statistics

To access client statistics, you need to call mysqli_get_client_stats(). The function call does not require any parameters.

The function returns an associative array that contains the name of the statistic as the key and the statistical data as the value.

Client statistics can also be accessed by calling the phpinfo() function.

Accessing Connection Statistics

To access connection statistics call mysqli_get_connection_stats(). This takes the database connection handle as the parameter.

The function returns an associative array that contains the name of the statistic as the key and the statistical data as the value.

Buffered and Unbuffered Result Sets

Result sets can be buffered or unbuffered. Using default settings, ext/mysql and ext/mysqli work with buffered result sets for normal (non prepared statement) queries. Buffered result sets are cached on the client. After the query execution all results are fetched from the MySQL Server and stored in a cache on the client. The big advantage of buffered result sets is that they allow the server to free all resources allocated to a result set, once the results have been fetched by the client.

Unbuffered result sets on the other hand are kept much longer on the server. If you want to reduce memory consumption on the client, but increase load on the server, use unbuffered results. If you experience a high server load and the figures for unbuffered result sets are high, you should consider moving the load to the clients. Clients typically scale better than servers. "Load" does not only refer to memory buffers - the server also needs to keep other resources open, for example file handles and threads, before a result set can be freed.

Prepared Statements use unbuffered result sets by default. However, you can use mysqli_stmt_store_result() to enable buffered result sets.

Statistics returned by MySQL Native Driver

The following tables show a list of statistics returned by the mysqli_get_client_stats() and mysqli_get_connection_stats() functions.

Returned mysqlnd statistics: Network
Statistic | Scope | Description | Notes
--- | --- | --- | ---
bytes_sent | Connection | Number of bytes sent from PHP to the MySQL server | Can be used to check the efficiency of the compression protocol
bytes_received | Connection | Number of bytes received from MySQL server | Can be used to check the efficiency of the compression protocol
packets_sent | Connection | Number of MySQL Client Server protocol packets sent | Used for debugging Client Server protocol implementation
packets_received | Connection | Number of MySQL Client Server protocol packets received | Used for debugging Client Server protocol implementation
protocol_overhead_in | Connection | MySQL Client Server protocol overhead in bytes for incoming traffic. Currently only the Packet Header (4 bytes) is considered as overhead. protocol_overhead_in = packets_received * 4 | Used for debugging Client Server protocol implementation
protocol_overhead_out | Connection | MySQL Client Server protocol overhead in bytes for outgoing traffic. Currently only the Packet Header (4 bytes) is considered as overhead. protocol_overhead_out = packets_sent * 4 | Used for debugging Client Server protocol implementation
bytes_received_ok_packet | Connection | Total size of bytes of MySQL Client Server protocol OK packets received. OK packets can contain a status message. The length of the status message can vary and thus the size of an OK packet is not fixed. | Used for debugging CS protocol implementation. Note that the total size in bytes includes the size of the header packet (4 bytes, see protocol overhead).
packets_received_ok | Connection | Number of MySQL Client Server protocol OK packets received. | Used for debugging CS protocol implementation. Note that the total size in bytes includes the size of the header packet (4 bytes, see protocol overhead).
bytes_received_eof_packet | Connection | Total size in bytes of MySQL Client Server protocol EOF packets received. EOF can vary in size depending on the server version. Also, EOF can transport an error message. | Used for debugging CS protocol implementation. Note that the total size in bytes includes the size of the header packet (4 bytes, see protocol overhead).
packets_received_eof | Connection | Number of MySQL Client Server protocol EOF packets. Like with other packet statistics the number of packets will be increased even if PHP does not receive the expected packet but, for example, an error message. | Used for debugging CS protocol implementation. Note that the total size in bytes includes the size of the header packet (4 bytes, see protocol overhead).
bytes_received_rset_header_packet | Connection | Total size in bytes of MySQL Client Server protocol result set header packets. The size of the packets varies depending on the payload (LOAD LOCAL INFILE, INSERT, UPDATE, SELECT, error message). | Used for debugging CS protocol implementation. Note that the total size in bytes includes the size of the header packet (4 bytes, see protocol overhead).
packets_received_rset_header | Connection | Number of MySQL Client Server protocol result set header packets. | Used for debugging CS protocol implementation. Note that the total size in bytes includes the size of the header packet (4 bytes, see protocol overhead).
bytes_received_rset_field_meta_packet | Connection | Total size in bytes of MySQL Client Server protocol result set meta data (field information) packets. Of course the size varies with the fields in the result set. The packet may also transport an error or an EOF packet in case of COM_LIST_FIELDS. | Only useful for debugging CS protocol implementation. Note that the total size in bytes includes the size of the header packet (4 bytes, see protocol overhead).
packets_received_rset_field_meta | Connection | Number of MySQL Client Server protocol result set meta data (field information) packets. | Only useful for debugging CS protocol implementation. Note that the total size in bytes includes the size of the header packet (4 bytes, see protocol overhead).
bytes_received_rset_row_packet | Connection | Total size in bytes of MySQL Client Server protocol result set row data packets. The packet may also transport an error or an EOF packet. You can reverse engineer the number of error and EOF packets by subtracting rows_fetched_from_server_normal and rows_fetched_from_server_ps from bytes_received_rset_row_packet. | Only useful for debugging CS protocol implementation. Note that the total size in bytes includes the size of the header packet (4 bytes, see protocol overhead).
packets_received_rset_row | Connection | Number of MySQL Client Server protocol result set row data packets and their total size in bytes. | Only useful for debugging CS protocol implementation. Note that the total size in bytes includes the size of the header packet (4 bytes, see protocol overhead).
bytes_received_prepare_response_packet | Connection | Total size in bytes of MySQL Client Server protocol OK for Prepared Statement Initialization packets (prepared statement init packets). The packet may also transport an error. The packet size depends on the MySQL version: 9 bytes with MySQL 4.1 and 12 bytes from MySQL 5.0 on. There is no safe way to know how many errors happened. You may be able to guess that an error has occurred if, for example, you always connect to MySQL 5.0 or newer and, bytes_received_prepare_response_packet != packets_received_prepare_response * 12. See also ps_prepared_never_executed, ps_prepared_once_executed. | Only useful for debugging CS protocol implementation. Note that the total size in bytes includes the size of the header packet (4 bytes, see protocol overhead).
packets_received_prepare_response | Connection | Number of MySQL Client Server protocol OK for Prepared Statement Initialization packets (prepared statement init packets). | Only useful for debugging CS protocol implementation. Note that the total size in bytes includes the size of the header packet (4 bytes, see protocol overhead).
bytes_received_change_user_packet | Connection | Total size in bytes of MySQL Client Server protocol COM_CHANGE_USER packets. The packet may also transport an error or EOF. | Only useful for debugging CS protocol implementation. Note that the total size in bytes includes the size of the header packet (4 bytes, see protocol overhead).
packets_received_change_user | Connection | Number of MySQL Client Server protocol COM_CHANGE_USER packets | Only useful for debugging CS protocol implementation. Note that the total size in bytes includes the size of the header packet (4 bytes, see protocol overhead).
packets_sent_command | Connection | Number of MySQL Client Server protocol commands sent from PHP to MySQL. There is no way to know which specific commands and how many of them have been sent. At its best you can use it to check if PHP has sent any commands to MySQL to know if you can consider to disable MySQL support in your PHP binary. There is also no way to reverse engineer the number of errors that may have occurred while sending data to MySQL. The only error that is recorded is command_buffer_too_small (see below). | Only useful for debugging CS protocol implementation.
bytes_received_real_data_normal | Connection | Number of bytes of payload fetched by the PHP client from mysqlnd using the text protocol. | This is the size of the actual data contained in result sets that do not originate from prepared statements and which have been fetched by the PHP client. Note that although a full result set may have been pulled from MySQL by mysqlnd, this statistic only counts actual data pulled from mysqlnd by the PHP client. An example of a code sequence that will increase the value is as follows:
    $mysqli = new mysqli();
    $res = $mysqli->query("SELECT 'abc'");
    $res->fetch_assoc();
    $res->close();
Every fetch operation will increase the value. The statistic will not be increased if the result set is only buffered on the client, but not fetched, such as in the following example:
    $mysqli = new mysqli();
    $res = $mysqli->query("SELECT 'abc'");
    $res->close();
This statistic is available as of PHP version 5.3.4.
bytes_received_real_data_ps | Connection | Number of bytes of the payload fetched by the PHP client from mysqlnd using the prepared statement protocol. | This is the size of the actual data contained in result sets that originate from prepared statements and which has been fetched by the PHP client. The value will not be increased if the result set is not subsequently read by the PHP client. Note that although a full result set may have been pulled from MySQL by mysqlnd, this statistic only counts actual data pulled from mysqlnd by the PHP client. See also bytes_received_real_data_normal. This statistic is available as of PHP version 5.3.4.

Result Set

Returned mysqlnd statistics: Result Set
Statistic | Scope | Description | Notes
--- | --- | --- | ---
result_set_queries | Connection | Number of queries that have generated a result set. Examples of queries that generate a result set: SELECT, SHOW. The statistic will not be incremented if there is an error reading the result set header packet from the line. | You may use it as an indirect measure for the number of queries PHP has sent to MySQL, for example, to identify a client that causes a high database load.
non_result_set_queries | Connection | Number of queries that did not generate a result set. Examples of queries that do not generate a result set: INSERT, UPDATE, LOAD DATA, SHOW. The statistic will not be incremented if there is an error reading the result set header packet from the line. | You may use it as an indirect measure for the number of queries PHP has sent to MySQL, for example, to identify a client that causes a high database load.
no_index_used | Connection | Number of queries that have generated a result set but did not use an index (see also mysqld start option –log-queries-not-using-indexes). If you want these queries to be reported you can use mysqli_report(MYSQLI_REPORT_INDEX) to make ext/mysqli throw an exception. If you prefer a warning instead of an exception use mysqli_report(MYSQLI_REPORT_INDEX ^ MYSQLI_REPORT_STRICT). |
bad_index_used | Connection | Number of queries that have generated a result set and did not use a good index (see also mysqld start option –log-slow-queries). | If you want these queries to be reported you can use mysqli_report(MYSQLI_REPORT_INDEX) to make ext/mysqli throw an exception. If you prefer a warning instead of an exception use mysqli_report(MYSQLI_REPORT_INDEX ^ MYSQLI_REPORT_STRICT)
slow_queries | Connection | SQL statements that took more than long_query_time seconds to execute and required at least min_examined_row_limit rows to be examined. | Not reported through mysqli_report()
buffered_sets | Connection | Number of buffered result sets returned by "normal" queries. "Normal" means "not prepared statement" in the following notes. | Examples of API calls that will buffer result sets on the client: mysql_query(), mysqli_query(), mysqli_store_result(), mysqli_stmt_get_result(). Buffering result sets on the client ensures that server resources are freed as soon as possible and it makes result set scrolling easier. The downside is the additional memory consumption on the client for buffering data. Note that mysqlnd (unlike the MySQL Client Library) respects the PHP memory limit because it uses PHP internal memory management functions to allocate memory. This is also the reason why memory_get_usage() reports a higher memory consumption when using mysqlnd instead of the MySQL Client Library. memory_get_usage() does not measure the memory consumption of the MySQL Client Library at all because the MySQL Client Library does not use PHP internal memory management functions monitored by the function!
unbuffered_sets | Connection | Number of unbuffered result sets returned by normal (non prepared statement) queries. | Examples of API calls that will not buffer result sets on the client: mysqli_use_result()
ps_buffered_sets | Connection | Number of buffered result sets returned by prepared statements. By default prepared statements are unbuffered. | Examples of API calls that will not buffer result sets on the client: mysqli_stmt_store_result
ps_unbuffered_sets | Connection | Number of unbuffered result sets returned by prepared statements. | By default prepared statements are unbuffered.
flushed_normal_sets | Connection | Number of result sets from normal (non prepared statement) queries with unread data which have been flushed silently for you. Flushing happens only with unbuffered result sets. | Unbuffered result sets must be fetched completely before a new query can be run on the connection otherwise MySQL will throw an error. If the application does not fetch all rows from an unbuffered result set, mysqlnd does implicitly fetch the result set to clear the line. See also rows_skipped_normal, rows_skipped_ps. Some possible causes for an implicit flush: Faulty client application Client stopped reading after it found what it was looking for but has made MySQL calculate more records than needed Client application has stopped unexpectedly
flushed_ps_sets | Connection | Number of result sets from prepared statements with unread data which have been flushed silently for you. Flushing happens only with unbuffered result sets. | Unbuffered result sets must be fetched completely before a new query can be run on the connection otherwise MySQL will throw an error. If the application does not fetch all rows from an unbuffered result set, mysqlnd does implicitly fetch the result set to clear the line. See also rows_skipped_normal, rows_skipped_ps. Some possible causes for an implicit flush: Faulty client application Client stopped reading after it found what it was looking for but has made MySQL calculate more records than needed Client application has stopped unexpectedly
ps_prepared_never_executed | Connection | Number of statements prepared but never executed. | Prepared statements occupy server resources. You should not prepare a statement if you do not plan to execute it.
ps_prepared_once_executed | Connection | Number of prepared statements executed only one. | One of the ideas behind prepared statements is that the same query gets executed over and over again (with different parameters) and some parsing and other preparation work can be saved, if statement execution is split up in separate prepare and execute stages. The idea is to prepare once and "cache" results, for example, the parse tree to be reused during multiple statement executions. If you execute a prepared statement only once the two stage processing can be inefficient compared to "normal" queries because all the caching means extra work and it takes (limited) server resources to hold the cached information. Consequently, prepared statements that are executed only once may cause performance hurts.
rows_fetched_from_server_normal, rows_fetched_from_server_ps | Connection | Total number of result set rows successfully fetched from MySQL regardless if the client application has consumed them or not. Some of the rows may not have been fetched by the client application but have been flushed implicitly. | See also packets_received_rset_row
rows_buffered_from_client_normal, rows_buffered_from_client_ps | Connection | Total number of successfully buffered rows originating from a "normal" query or a prepared statement. This is the number of rows that have been fetched from MySQL and buffered on client. Note that there are two distinct statistics on rows that have been buffered (MySQL to mysqlnd internal buffer) and buffered rows that have been fetched by the client application (mysqlnd internal buffer to client application). If the number of buffered rows is higher than the number of fetched buffered rows it can mean that the client application runs queries that cause larger result sets than needed resulting in rows not read by the client. | Examples of queries that will buffer results: mysqli_query(), mysqli_store_result()
rows_fetched_from_client_normal_buffered, rows_fetched_from_client_ps_buffered | Connection | Total number of rows fetched by the client from a buffered result set created by a normal query or a prepared statement. |
rows_fetched_from_client_normal_unbuffered, rows_fetched_from_client_ps_unbuffered | Connection | Total number of rows fetched by the client from a unbuffered result set created by a "normal" query or a prepared statement. |
rows_fetched_from_client_ps_cursor | Connection | Total number of rows fetch by the client from a cursor created by a prepared statement. |
rows_skipped_normal, rows_skipped_ps | Connection | Reserved for future use (currently not supported) |
copy_on_write_saved, copy_on_write_performed | Process | With mysqlnd, variables returned by the extensions point into mysqlnd internal network result buffers. If you do not change the variables, fetched data will be kept only once in memory. If you change the variables, mysqlnd has to perform a copy-on-write to protect the internal network result buffers from being changed. With the MySQL Client Library you always hold fetched data twice in memory. Once in the internal MySQL Client Library buffers and once in the variables returned by the extensions. In theory mysqlnd can save up to 40% memory. However, note that the memory saving cannot be measured using memory_get_usage(). |
explicit_free_result, implicit_free_result | Connection, Process (only during prepared statement cleanup) | Total number of freed result sets. | The free is always considered explicit but for result sets created by an init command, for example, mysqli_options(MYSQLI_INIT_COMMAND , ...)
proto_text_fetched_null, proto_text_fetched_bit, proto_text_fetched_tinyint proto_text_fetched_short, proto_text_fetched_int24, proto_text_fetched_int proto_text_fetched_bigint, proto_text_fetched_decimal, proto_text_fetched_float proto_text_fetched_double, proto_text_fetched_date, proto_text_fetched_year proto_text_fetched_time, proto_text_fetched_datetime, proto_text_fetched_timestamp proto_text_fetched_string, proto_text_fetched_blob, proto_text_fetched_enum proto_text_fetched_set, proto_text_fetched_geometry, proto_text_fetched_other | Connection | Total number of columns of a certain type fetched from a normal query (MySQL text protocol). | Mapping from C API / MySQL meta data type to statistics name: MYSQL_TYPE_NULL - proto_text_fetched_null MYSQL_TYPE_BIT - proto_text_fetched_bit MYSQL_TYPE_TINY - proto_text_fetched_tinyint MYSQL_TYPE_SHORT - proto_text_fetched_short MYSQL_TYPE_INT24 - proto_text_fetched_int24 MYSQL_TYPE_LONG - proto_text_fetched_int MYSQL_TYPE_LONGLONG - proto_text_fetched_bigint MYSQL_TYPE_DECIMAL, MYSQL_TYPE_NEWDECIMAL - proto_text_fetched_decimal MYSQL_TYPE_FLOAT - proto_text_fetched_float MYSQL_TYPE_DOUBLE - proto_text_fetched_double MYSQL_TYPE_DATE, MYSQL_TYPE_NEWDATE - proto_text_fetched_date MYSQL_TYPE_YEAR - proto_text_fetched_year MYSQL_TYPE_TIME - proto_text_fetched_time MYSQL_TYPE_DATETIME - proto_text_fetched_datetime MYSQL_TYPE_TIMESTAMP - proto_text_fetched_timestamp MYSQL_TYPE_STRING, MYSQL_TYPE_VARSTRING, MYSQL_TYPE_VARCHAR - proto_text_fetched_string MYSQL_TYPE_TINY_BLOB, MYSQL_TYPE_MEDIUM_BLOB, MYSQL_TYPE_LONG_BLOB, MYSQL_TYPE_BLOB - proto_text_fetched_blob MYSQL_TYPE_ENUM - proto_text_fetched_enum MYSQL_TYPE_SET - proto_text_fetched_set MYSQL_TYPE_GEOMETRY - proto_text_fetched_geometry Any MYSQL_TYPE_* not listed before (there should be none) - proto_text_fetched_other Note that the MYSQL_*-type constants may not be associated with the very same SQL column types in every version of MySQL.
proto_binary_fetched_null, proto_binary_fetched_bit, proto_binary_fetched_tinyint proto_binary_fetched_short, proto_binary_fetched_int24, proto_binary_fetched_int, proto_binary_fetched_bigint, proto_binary_fetched_decimal, proto_binary_fetched_float, proto_binary_fetched_double, proto_binary_fetched_date, proto_binary_fetched_year, proto_binary_fetched_time, proto_binary_fetched_datetime, proto_binary_fetched_timestamp, proto_binary_fetched_string, proto_binary_fetched_blob, proto_binary_fetched_enum, proto_binary_fetched_set, proto_binary_fetched_geometry, proto_binary_fetched_other | Connection | Total number of columns of a certain type fetched from a prepared statement (MySQL binary protocol). | For type mapping see proto_text_* described in the preceding text.

Returned mysqlnd statistics: Connection
Statistic | Scope | Description | Notes
--- | --- | --- | ---
connect_success, connect_failure | Connection | Total number of successful / failed connection attempt. | Reused connections and all other kinds of connections are included.
reconnect | Process | Total number of (real_)connect attempts made on an already opened connection handle. | The code sequence $link = new mysqli(...); $link->real_connect(...) will cause a reconnect. But $link = new mysqli(...); $link->connect(...) will not because $link->connect(...) will explicitly close the existing connection before a new connection is established.
pconnect_success | Connection | Total number of successful persistent connection attempts. | Note that connect_success holds the sum of successful persistent and non-persistent connection attempts. The number of successful non-persistent connection attempts is connect_success - pconnect_success.
active_connections | Connection | Total number of active persistent and non-persistent connections. |
active_persistent_connections | Connection | Total number of active persistent connections. | The total number of active non-persistent connections is active_connections - active_persistent_connections.
explicit_close | Connection | Total number of explicitly closed connections (ext/mysqli only). | Examples of code snippets that cause an explicit close :
    $link = new mysqli(...); $link->close(...)
    $link = new mysqli(...); $link->connect(...)
implicit_close | Connection | Total number of implicitly closed connections (ext/mysqli only). | Examples of code snippets that cause an implicit close : $link = new mysqli(...); $link->real_connect(...) unset($link) Persistent connection: pooled connection has been created with real_connect and there may be unknown options set - close implicitly to avoid returning a connection with unknown options Persistent connection: ping/change_user fails and ext/mysqli closes the connection end of script execution: close connections that have not been closed by the user
disconnect_close | Connection | Connection failures indicated by the C API call mysql_real_connect() during an attempt to establish a connection. | It is called disconnect_close because the connection handle passed to the C API call will be closed.
in_middle_of_command_close | Process | A connection has been closed in the middle of a command execution (outstanding result sets not fetched, after sending a query and before retrieving an answer, while fetching data, while transferring data with LOAD DATA). | Unless you use asynchronous queries this should only happen if your script stops unexpectedly and PHP shuts down the connections for you.
init_command_executed_count | Connection | Total number of init command executions, for example, mysqli_options(MYSQLI_INIT_COMMAND , ...). | The number of successful executions is init_command_executed_count - init_command_failed_count.
init_command_failed_count | Connection | Total number of failed init commands. |

Returned mysqlnd statistics: COM_* Command
Statistic | Scope | Description | Notes
--- | --- | --- | ---
com_quit, com_init_db, com_query, com_field_list, com_create_db, com_drop_db, com_refresh, com_shutdown, com_statistics, com_process_info, com_connect, com_process_kill, com_debug, com_ping, com_time, com_delayed_insert, com_change_user, com_binlog_dump, com_table_dump, com_connect_out, com_register_slave, com_stmt_prepare, com_stmt_execute, com_stmt_send_long_data, com_stmt_close, com_stmt_reset, com_stmt_set_option, com_stmt_fetch, com_daemon | Connection | Total number of attempts to send a certain COM_* command from PHP to MySQL. | The statistics are incremented after checking the line and immediately before sending the corresponding MySQL client server protocol packet. If mysqlnd fails to send the packet over the wire the statistics will not be decremented. In case of a failure mysqlnd emits a PHP warning "Error while sending %s packet. PID=%d." Usage examples: Check if PHP sends certain commands to MySQL, for example, check if a client sends COM_PROCESS_KILL Calculate the average number of prepared statement executions by comparing COM_EXECUTE with COM_PREPARE Check if PHP has run any non-prepared SQL statements by checking if COM_QUERY is zero Identify PHP scripts that run an excessive number of SQL statements by checking COM_QUERY and COM_EXECUTE

Miscellaneous

Returned mysqlnd statistics: Miscellaneous
Statistic | Scope | Description | Notes
--- | --- | --- | ---
explicit_stmt_close, implicit_stmt_close | Process | Total number of close prepared statements. | A close is always considered explicit but for a failed prepare.
mem_emalloc_count, mem_emalloc_ammount, mem_ecalloc_count, mem_ecalloc_ammount, mem_erealloc_count, mem_erealloc_ammount, mem_efree_count, mem_malloc_count, mem_malloc_ammount, mem_calloc_count, mem_calloc_ammount, mem_realloc_count, mem_realloc_ammount, mem_free_count | Process | Memory management calls. | Development only.
command_buffer_too_small | Connection | Number of network command buffer extensions while sending commands from PHP to MySQL. | mysqlnd allocates an internal command/network buffer of mysqlnd.net_cmd_buffer_size (php.ini) bytes for every connection. If a MySQL Client Server protocol command, for example, COM_QUERY (normal query), does not fit into the buffer, mysqlnd will grow the buffer to what is needed for sending the command. Whenever the buffer gets extended for one connection command_buffer_too_small will be incremented by one. If mysqlnd has to grow the buffer beyond its initial size of mysqlnd.net_cmd_buffer_size (php.ini) bytes for almost every connection, you should consider to increase the default size to avoid re-allocations. The default buffer size is 2048 bytes in PHP 5.3.0. In future versions the default will be 4kB or larger. The default can changed either through the php.ini setting mysqlnd.net_cmd_buffer_size or using mysqli_options(MYSQLI_OPT_NET_CMD_BUFFER_SIZE, int size). It is recommended to set the buffer size to no less than 4096 bytes because mysqlnd also uses it when reading certain communication packet from MySQL. In PHP 5.3.0, mysqlnd will not grow the buffer if MySQL sends a packet that is larger than the current size of the buffer. As a consequence mysqlnd is unable to decode the packet and the client application will get an error. There are only two situations when the packet can be larger than the 2048 bytes default of mysqlnd.net_cmd_buffer_size in PHP 5.3.0: the packet transports a very long error message or the packet holds column meta data from COM_LIST_FIELD (mysql_list_fields()) and the meta data comes from a string column with a very long default value (>1900 bytes). No bug report on this exists - it should happen rarely. As of PHP 5.3.2 mysqlnd does not allow setting buffers smaller than 4096 bytes.
connection_reused |  |  |

Persistent Connections
Notes
Mysqlnd
PHP Manual