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131 lines
4.2 KiB
C++
131 lines
4.2 KiB
C++
#pragma once
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#include <Common/Arena.h>
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/**
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* In some aggregation scenarios, when adding a key to the hash table, we
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* start with a temporary key object, and if it turns out to be a new key,
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* we must make it persistent (e.g. copy to an Arena) and use the resulting
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* persistent object as hash table key. This happens only for StringRef keys,
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* because other key types are stored by value, but StringRef is a pointer-like
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* type: the actual data are stored elsewhere. Even for StringRef, we don't
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* make a persistent copy of the key in each of the following cases:
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* 1) the aggregation method doesn't use temporary keys, so they're persistent
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* from the start;
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* 1) the key is already present in the hash table;
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* 3) that particular key is stored by value, e.g. a short StringRef key in
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* StringHashMap.
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*
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* In the past, the caller was responsible for making the key persistent after
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* in was inserted. emplace() returned whether the key is new or not, so the
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* caller only stored new keys (this is case (2) from the above list). However,
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* now we are adding a compound hash table for StringRef keys, so case (3)
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* appears. The decision about persistence now depends on some properties of
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* the key, and the logic of this decision is tied to the particular hash table
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* implementation. This means that the hash table user now doesn't have enough
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* data and logic to make this decision by itself.
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*
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* To support these new requirements, we now manage key persistence by passing
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* a special key holder to emplace(), which has the functions to make the key
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* persistent or to discard it. emplace() then calls these functions at the
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* appropriate moments.
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*
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* This approach has the following benefits:
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* - no extra runtime branches in the caller to make the key persistent.
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* - no additional data is stored in the hash table itself, which is important
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* when it's used in aggregate function states.
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* - no overhead when the key memory management isn't needed: we just pass the
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* bare key without any wrapper to emplace(), and the default callbacks do
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* nothing.
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*
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* This file defines the default key persistence functions, as well as two
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* different key holders and corresponding functions for storing StringRef
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* keys to Arena.
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*/
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/**
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* Returns the key. Can return the temporary key initially.
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* After the call to keyHolderPersistKey(), must return the persistent key.
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*/
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template <typename Key>
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inline Key & ALWAYS_INLINE keyHolderGetKey(Key && key) { return key; }
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/**
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* Make the key persistent. keyHolderGetKey() must return the persistent key
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* after this call.
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*/
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template <typename Key>
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inline void ALWAYS_INLINE keyHolderPersistKey(Key &&) {}
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/**
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* Discard the key. Calling keyHolderGetKey() is ill-defined after this.
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*/
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template <typename Key>
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inline void ALWAYS_INLINE keyHolderDiscardKey(Key &&) {}
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namespace DB
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{
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/**
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* ArenaKeyHolder is a key holder for hash tables that serializes a StringRef
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* key to an Arena.
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*/
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struct ArenaKeyHolder
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{
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StringRef key;
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Arena & pool;
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};
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}
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inline StringRef & ALWAYS_INLINE keyHolderGetKey(DB::ArenaKeyHolder & holder)
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{
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return holder.key;
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}
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inline void ALWAYS_INLINE keyHolderPersistKey(DB::ArenaKeyHolder & holder)
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{
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// Hash table shouldn't ask us to persist a zero key
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assert(holder.key.size > 0);
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holder.key.data = holder.pool.insert(holder.key.data, holder.key.size);
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}
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inline void ALWAYS_INLINE keyHolderDiscardKey(DB::ArenaKeyHolder &)
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{
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}
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namespace DB
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{
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/**
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* SerializedKeyHolder is a key holder for a StringRef key that is already
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* serialized to an Arena. The key must be the last allocation in this Arena,
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* and is discarded by rolling back the allocation.
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*/
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struct SerializedKeyHolder
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{
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StringRef key;
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Arena & pool;
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};
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}
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inline StringRef & ALWAYS_INLINE keyHolderGetKey(DB::SerializedKeyHolder & holder)
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{
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return holder.key;
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}
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inline void ALWAYS_INLINE keyHolderPersistKey(DB::SerializedKeyHolder &)
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{
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}
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inline void ALWAYS_INLINE keyHolderDiscardKey(DB::SerializedKeyHolder & holder)
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{
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[[maybe_unused]] void * new_head = holder.pool.rollback(holder.key.size);
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assert(new_head == holder.key.data);
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holder.key.data = nullptr;
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holder.key.size = 0;
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}
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