LinkedImmutableString Class Reference

LinkedImmutableString is designed for storing rows (values) in hash join. More...

#include <immutable_string.h>

Classes
struct	Decoded

Public Member Functions
	LinkedImmutableString (const char *encoded)
	NOTE: nullptr is a legal value for encoded, and signals the same thing as nullptr would on a const char *. More...
Decoded	Decode () const
bool	operator== (std::nullptr_t) const
bool	operator!= (std::nullptr_t) const

Static Public Member Functions
static LinkedImmutableString	EncodeHeader (LinkedImmutableString next, char **dst)
	Encode the given string and “next” pointer as a header for LinkedImmutableString, and returns a new object pointing to it. More...
static size_t	RequiredBytesForEncode (size_t length)
	Calculates an upper bound on the space required for encoding a string of the given length. More...

Private Attributes
const char *	m_ptr

Detailed Description

LinkedImmutableString is designed for storing rows (values) in hash join.

It does not need a length, since it is implicit from the contents; however, since there might be multiple values with the same key, we simulate a multimap by having a “next” pointer. (Normally, linked lists are a bad idea due to pointer chasing, but here, we're doing so much work for each value that the overhead disappears into the noise.)

As the next pointer is usually be very close in memory to ourselves (nearly all rows are stored in the same MEM_ROOT), we don't need to store the entire pointer; instead, we store the difference between the start of this string and the next pointer, as a zigzag-encoded Varint128. As with the length in ImmutableStringWithLength, this typically saves 6–7 bytes for each value. The special value of 0 means that there is no next pointer (ie., it is nullptr), as that would otherwise be an infinite loop.

Constructor & Destructor Documentation

LinkedImmutableString()

LinkedImmutableString::LinkedImmutableString ( const char *  encoded )

inlineexplicit

NOTE: nullptr is a legal value for encoded, and signals the same thing as nullptr would on a const char *.

Member Function Documentation

Decode()

LinkedImmutableString::Decoded LinkedImmutableString::Decode ( ) const

inline

EncodeHeader()

LinkedImmutableString LinkedImmutableString::EncodeHeader ( LinkedImmutableString  next, char **  dst  )

inlinestatic

Encode the given string and “next” pointer as a header for LinkedImmutableString, and returns a new object pointing to it.

Note that unlike ImmutableStringWithLength::Encode(), this only encodes the header; since there is no explicitly stored length, you must write the contents of the string yourself.

*dst must contain at least the number of bytes returned by RequiredBytesForEncode. It is moved to one byte past the end of the written stream (which is the right place to store the string itself).

operator!=()

bool LinkedImmutableString::operator!= ( std::nullptr_t  ) const

inline

operator==()

bool LinkedImmutableString::operator== ( std::nullptr_t  ) const

inline

RequiredBytesForEncode()

static size_t LinkedImmutableString::RequiredBytesForEncode ( size_t  length )

inlinestatic

Calculates an upper bound on the space required for encoding a string of the given length.

Member Data Documentation

m_ptr

const char* LinkedImmutableString::m_ptr

private

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The documentation for this class was generated from the following file:

• sql/immutable_string.h