Dynamic Columns API

This page describes client-side of MariaDB 10.0.1 API and MariaDB Connector/C 2.0 for reading and writing Dynamic Columns blobs.

Normally, you should use Dynamic column functions which are run inside the MariaDB server and allow one to access Dynamic Columns content without any client-side libraries.

If you need to read/write dynamic column blobs on the client for some reason, this API enables that.

Where to get it

The API is a part of libmysql C client library. In order to use it, one needs to include this header file

#include <mysql/ma_dyncol.h>

and link against libmysql.

Data structures


DYNAMIC_COLUMN represents a packed dynamic column blob. It is essentially a string-with-length and is defined as follows:

/* A generic-purpose arbitrary-length string defined in MySQL Client API */
typedef struct st_dynamic_string
  char *str;
  size_t length,max_length,alloc_increment;




Dynamic columns blob stores {name, value} pairs. DYNAMIC_COLUMN_VALUE structure is used to represent the value in accessible form.

struct st_dynamic_column_value
    long long long_value;
    unsigned long long ulong_value;
    double double_value;
    struct {
      MYSQL_LEX_STRING value;
      CHARSET_INFO *charset;
    } string;
    struct {
      decimal_digit_t buffer[DECIMAL_BUFF_LENGTH];
      decimal_t value;
    } decimal;
    MYSQL_TIME time_value;
  } x;
typedef struct st_dynamic_column_value DYNAMIC_COLUMN_VALUE;

Every value has a type, which is determined by the type member.

typestructure field
DYN_COL_STRINGvalue.x.string.value, value.x.string.charset


  • Values with type DYN_COL_NULL do not ever occur in dynamic columns blobs.
  • Type DYN_COL_DYNCOL means that the value is a packed dynamic blob. This is how nested dynamic columns are done.
  • Before storing a value to value.x.decimal.value, one must call mariadb_dyncol_prepare_decimal() to initialize the space for storage.


enum enum_dyncol_func_result is used as return value.

0ER_DYNCOL_NO(the same as ER_DYNCOL_OK but for functions which return a YES/NO)
1ER_DYNCOL_YESYES response or success
2ER_DYNCOL_TRUNCATEDOperation succeeded but the data was truncated
-1ER_DYNCOL_FORMATWrong format of the encoded string
-2ER_DYNCOL_LIMITA limit of implementation reached
-3ER_DYNCOL_RESOURCEOut of resources
-4ER_DYNCOL_DATAIncorrect input data
-5ER_DYNCOL_UNKNOWN_CHARSETUnknown character set

Result codes that are less than zero represent error conditions.

Function reference

Functions come in pairs:

  • xxx_num() operates on the old (pre-MariaDB-10.0.1) dynamic column blob format where columns were identified by numbers.
  • xxx_named() can operate on both old or new data format. If it modifies the blob, it will convert it to the new data format.

You should use xxx_named() functions, unless you need to keep the data compatible with MariaDB versions before 10.0.1.


  1. define mariadb_dyncol_init(A) memset((A), 0, sizeof(*(A)))

It is correct initialization for an empty packed dynamic blob.


void mariadb_dyncol_free(DYNAMIC_COLUMN *str);


strINPacked dynamic blob which memory should be freed.

mariadb_dyncol_create_many (num|named)

Create a packed dynamic blob from arrays of values and names.

enum enum_dyncol_func_result
mariadb_dyncol_create_many_num(DYNAMIC_COLUMN *str,
                               uint column_count,
                               uint *column_numbers,
                               DYNAMIC_COLUMN_VALUE *values,
                               my_bool new_string);
enum enum_dyncol_func_result
mariadb_dyncol_create_many_named(DYNAMIC_COLUMN *str,
                                 uint column_count,
                                 MYSQL_LEX_STRING *column_keys,
                                 DYNAMIC_COLUMN_VALUE *values,
                                 my_bool new_string);


strOUTPacked dynamic blob will be put here
column_countINNumber of columns
column_numbersINColumn numbers array (old format)
column_keysINColumn names array (new format)
valuesINColumn values array
new_stringINIf TRUE then the str will be reinitialized (not freed) before usage

mariadb_dyncol_update_many (num|named)

Add or update columns in a dynamic columns blob. To delete a column, update its value to a "non-value" of type DYN_COL_NULL

enum enum_dyncol_func_result
mariadb_dyncol_update_many_num(DYNAMIC_COLUMN *str,
                               uint column_count,
                               uint *column_numbers,
                               DYNAMIC_COLUMN_VALUE *values);
enum enum_dyncol_func_result
mariadb_dyncol_update_many_named(DYNAMIC_COLUMN *str,
                                 uint column_count,
                                 MYSQL_LEX_STRING *column_keys,
                                 DYNAMIC_COLUMN_VALUE *values);
strIN/OUTDynamic columns blob to be modified.
column_countINNumber of columns in following arrays
column_numbersINColumn numbers array (old format)
column_keysINColumn names array (new format)
valuesINColumn values array

mariadb_dyncol_exists (num|named)

Check if column with given name exists in the blob

enum enum_dyncol_func_result
mariadb_dyncol_exists_num(DYNAMIC_COLUMN *str, uint column_number);
enum enum_dyncol_func_result
mariadb_dyncol_exists_named(DYNAMIC_COLUMN *str, MYSQL_LEX_STRING *column_key);
strINPacked dynamic columns string.
column_numberINColumn number (old format)
column_keyINColumn name (new format)

The function returns YES/NO or Error code


Get number of columns in a dynamic column blob

enum enum_dyncol_func_result
mariadb_dyncol_column_count(DYNAMIC_COLUMN *str, uint *column_count);
strINPacked dynamic columns string.
column_countOUTNumber of not NULL columns in the dynamic columns string

mariadb_dyncol_list (num|named)

List columns in a dynamic column blob.

enum enum_dyncol_func_result
mariadb_dyncol_list_num(DYNAMIC_COLUMN *str, uint *column_count, uint **column_numbers);
enum enum_dyncol_func_result
mariadb_dyncol_list_named(DYNAMIC_COLUMN *str, uint *column_count,
                          MYSQL_LEX_STRING **column_keys);
strINPacked dynamic columns string.
column_countOUTNumber of columns in following arrays
column_numbersOUTColumn numbers array (old format). Caller should free this array.
column_keysOUTColumn names array (new format). Caller should free this array.

mariadb_dyncol_get (num|named)

Get a value of one column

enum enum_dyncol_func_result
mariadb_dyncol_get_num(DYNAMIC_COLUMN *org, uint column_number,
                       DYNAMIC_COLUMN_VALUE *value);
enum enum_dyncol_func_result
mariadb_dyncol_get_named(DYNAMIC_COLUMN *str, MYSQL_LEX_STRING *column_key,
                         DYNAMIC_COLUMN_VALUE *value);
strINPacked dynamic columns string.
column_numberINColumn numbers array (old format)
column_keyINColumn names array (new format)
valueOUTValue of the column

If the column is not found NULL returned as a value of the column.


Get value of all columns

enum enum_dyncol_func_result
mariadb_dyncol_unpack(DYNAMIC_COLUMN *str,
                      uint *column_count,
                      MYSQL_LEX_STRING **column_keys,
                      DYNAMIC_COLUMN_VALUE **values);
strINPacked dynamic columns string to unpack.
column_countOUTNumber of columns in following arrays
column_keysOUTColumn names array (should be free by caller)
valuesOUTValues of the columns array (should be free by caller)


Check whether the dynamic columns blob uses new data format (the one where columns are identified by names)

my_bool mariadb_dyncol_has_names(DYNAMIC_COLUMN *str);
strINPacked dynamic columns string.


Check whether dynamic column blob has correct data format.

enum enum_dyncol_func_result
mariadb_dyncol_check(DYNAMIC_COLUMN *str);
strINPacked dynamic columns string.


Get contents od a dynamic columns blob in a JSON form

enum enum_dyncol_func_result
mariadb_dyncol_json(DYNAMIC_COLUMN *str, DYNAMIC_STRING *json);
strINPacked dynamic columns string.
jsonOUTJSON representation

mariadb_dyncol_json() allocates memory for the parameter json which must be explicitly freed by the mariadb_dyncol_free() function to prevent memory leakage.


Get dynamic column value as one of the base types

enum enum_dyncol_func_result
mariadb_dyncol_val_str(DYNAMIC_STRING *str, DYNAMIC_COLUMN_VALUE *val,
                       CHARSET_INFO *cs, my_bool quote);
enum enum_dyncol_func_result
mariadb_dyncol_val_long(longlong *ll, DYNAMIC_COLUMN_VALUE *val);
enum enum_dyncol_func_result
mariadb_dyncol_val_double(double *dbl, DYNAMIC_COLUMN_VALUE *val);
str or ll or dblOUTvalue of the column


Initialize DYNAMIC_COLUMN_VALUE before value of value.x.decimal.value can be set

void mariadb_dyncol_prepare_decimal(DYNAMIC_COLUMN_VALUE *value);
valueOUTValue of the column

This function links value.x.decimal.value to value.x.decimal.buffer.


Initialize a DYNAMIC_COLUMN_VALUE structure to a safe default.

#define mariadb_dyncol_value_init(V) (V)->type= DYN_COL_NULL


Compare two column names for equality

int mariadb_dyncol_column_cmp_named(const MYSQL_LEX_STRING *s1,
                                    const MYSQL_LEX_STRING *s2);


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