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How variable-length fields work
Suppose a field can hold names up to a specified maximum. One record might contain “Li” and another “Alexandra.” A variable-length field can represent each value according to its actual length rather than treating both as values of the same fixed width. The maximum still matters: “variable length” does not mean unlimited.
Conceptually, the stored value needs both its content and a way to identify its length:
[length information][actual content]
This is only a conceptual model. A database may use length bytes, other metadata, padding, or additional storage mechanisms; there is no single physical layout shared by all systems.
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Variable-length versus fixed-length fields
| Feature | Variable-length field | Fixed-length field |
|---|---|---|
| Value size | Can differ between records, within the type’s limit | Uses a declared width |
| Short values | May use storage based on the actual content plus length information | May be padded or occupy the fixed width, depending on the database |
| Length metadata | Needs a way to represent the value’s length | May not need per-value length metadata |
| Storage and performance | Can help when value lengths vary widely, but metadata and row layout matter | Can suit fixed-width data; behavior depends on the implementation |
VARCHAR is a common variable-length character type, while CHAR commonly represents fixed-width character data. Those names are useful starting points, not guarantees of identical behavior across database products. Consult the documentation for the specific engine and type.
Does a variable-length field save space or run faster?
It can save space when values are often much shorter than their maximum, because the system may avoid reserving the full width for every value. But length information has a cost, and row layout, character encoding, page size, and the distribution of values can affect the result. IBM Informix 12.10 says its documented varying-length types can conserve disk space when lengths vary widely and notes that more compact tables can make queries faster; that is product-specific guidance, not a universal performance guarantee.
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Do not assume that VARCHAR is always smaller or faster than CHAR. The answer depends on the database, column definition, storage format, and workload.
How database implementations differ
IBM Informix 12.10
For the documented CHARACTER VARYING, VARCHAR, and related types, Informix stores the actual contents with a one-byte length field. Its documentation gives a limit of 254 bytes for indexed columns and 255 bytes for non-indexed columns in this type family. These byte limits are specific to the documented Informix version and types; they should not be applied to other products or interpreted automatically as character counts. IBM Informix: Character Varying Data.
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MySQL 9.7 InnoDB
In InnoDB’s COMPACT row format, variable-length columns use one- or two-byte length information depending on conditions such as the column’s maximum and actual lengths and whether data is stored externally. In DYNAMIC row format, long VARCHAR, VARBINARY, BLOB, and TEXT values can be kept fully off-page in applicable cases. Whether a value is stored off-page depends on page size and total row size. These are InnoDB storage details, not part of the general definition of a variable-length field. MySQL 9.7: InnoDB Row Formats.
MySQL 9.6 server implementation
MySQL’s server developer reference describes a variable-length string field as having one or two length bytes, relevant character bytes, and possible unused padding up to the column’s full length. Its copy routine copies the length bytes and relevant content bytes. This describes a particular server implementation and should not be taken as the physical layout of every SQL variable-length column. MySQL 9.6 server developer reference: sql/field_conv.cc.
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PostgreSQL C interfaces
PostgreSQL’s C-function documentation says variable-length types passed through that interface begin with an opaque four-byte length field and directs developers to set it with SET_VARSIZE. This is an internal C representation, not a general promise about how SQL VARCHAR columns are laid out. PostgreSQL 16: C-Language Functions.
PostgreSQL’s user-defined-type documentation describes a standard layout and macros for variable-length internal types, and says types whose internal values vary in size are usually desirable to make TOAST-able. That guidance concerns PostgreSQL type implementation. PostgreSQL 17: User-Defined Types.
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Oracle Database 19c
Oracle distinguishes SQL column types from programming-interface host variables. Its Pro*C/C++ documentation describes a VARCHAR host-variable structure with a two-byte length field before its string field. Oracle’s SQL VARCHAR2 type is separately described as variable-length character data, with limits and semantics that depend on context. The host-variable structure is not a description of the universal on-disk layout of an Oracle column. Oracle Database 19c Pro*C/C++: Datatypes and Host Variables.
Quick Recap
What to check when choosing a field type
- Maximum size: Check the limit for the exact database, type, and column context.
- Bytes versus characters: A byte limit may not equal a character count, particularly with multibyte character encodings.
- Storage behavior: Find out whether short values are padded, how length metadata is represented, and whether large values can move off-page.
- Workload: Consider the actual distribution of value lengths and the effects of row format and indexing rather than assuming variable length guarantees better storage or speed.
- Interface boundary: Keep SQL column behavior separate from internal C representations or host-variable layouts.
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