mirror of https://github.com/postgres/postgres
Tuples can have type RECORDOID and a typmod number that identifies a blessed TupleDesc in a backend-private cache. To support the sharing of such tuples through shared memory and temporary files, provide a typmod registry in shared memory. To achieve that, introduce per-session DSM segments, created on demand when a backend first runs a parallel query. The per-session DSM segment has a table-of-contents just like the per-query DSM segment, and initially the contents are a shared record typmod registry and a DSA area to provide the space it needs to grow. State relating to the current session is accessed via a Session object reached through global variable CurrentSession that may require significant redesign further down the road as we figure out what else needs to be shared or remodelled. Author: Thomas Munro Reviewed-By: Andres Freund Discussion: https://postgr.es/m/CAEepm=0ZtQ-SpsgCyzzYpsXS6e=kZWqk3g5Ygn3MDV7A8dabUA@mail.gmail.compull/31/head^2
Andres Freund
8 years ago
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/*-------------------------------------------------------------------------
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* |
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* session.c |
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* Encapsulation of user session. |
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* |
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* This is intended to contain data that needs to be shared between backends |
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* performing work for a client session. In particular such a session is |
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* shared between the leader and worker processes for parallel queries. At |
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* some later point it might also become useful infrastructure for separating |
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* backends from client connections, e.g. for the purpose of pooling. |
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* |
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* Currently this infrastructure is used to share: |
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* - typemod registry for ephemeral row-types, i.e. BlessTupleDesc etc. |
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* |
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* Portions Copyright (c) 2017, PostgreSQL Global Development Group |
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* |
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* src/backend/access/common/session.c |
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* |
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*------------------------------------------------------------------------- |
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*/ |
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#include "postgres.h" |
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#include "access/session.h" |
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#include "storage/lwlock.h" |
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#include "storage/shm_toc.h" |
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#include "utils/memutils.h" |
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#include "utils/typcache.h" |
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/* Magic number for per-session DSM TOC. */ |
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#define SESSION_MAGIC 0xabb0fbc9 |
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/*
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* We want to create a DSA area to store shared state that has the same |
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* lifetime as a session. So far, it's only used to hold the shared record |
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* type registry. We don't want it to have to create any DSM segments just |
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* yet in common cases, so we'll give it enough space to hold a very small |
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* SharedRecordTypmodRegistry. |
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*/ |
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#define SESSION_DSA_SIZE 0x30000 |
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/*
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* Magic numbers for state sharing in the per-session DSM area. |
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*/ |
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#define SESSION_KEY_DSA UINT64CONST(0xFFFFFFFFFFFF0001) |
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#define SESSION_KEY_RECORD_TYPMOD_REGISTRY UINT64CONST(0xFFFFFFFFFFFF0002) |
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/* This backend's current session. */ |
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Session *CurrentSession = NULL; |
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/*
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* Set up CurrentSession to point to an empty Session object. |
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*/ |
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void |
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InitializeSession(void) |
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{ |
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CurrentSession = MemoryContextAllocZero(TopMemoryContext, sizeof(Session)); |
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} |
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/*
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* Initialize the per-session DSM segment if it isn't already initialized, and |
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* return its handle so that worker processes can attach to it. |
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* |
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* Unlike the per-context DSM segment, this segement and its contents are |
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* reused for future parallel queries. |
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* |
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* Return DSM_HANDLE_INVALID if a segment can't be allocated due to lack of |
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* resources. |
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*/ |
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dsm_handle |
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GetSessionDsmHandle(void) |
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{ |
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shm_toc_estimator estimator; |
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shm_toc *toc; |
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dsm_segment *seg; |
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size_t typmod_registry_size; |
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size_t size; |
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void *dsa_space; |
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void *typmod_registry_space; |
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dsa_area *dsa; |
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MemoryContext old_context; |
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/*
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* If we have already created a session-scope DSM segment in this backend, |
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* return its handle. The same segment will be used for the rest of this |
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* backend's lifetime. |
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*/ |
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if (CurrentSession->segment != NULL) |
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return dsm_segment_handle(CurrentSession->segment); |
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/* Otherwise, prepare to set one up. */ |
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old_context = MemoryContextSwitchTo(TopMemoryContext); |
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shm_toc_initialize_estimator(&estimator); |
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/* Estimate space for the per-session DSA area. */ |
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shm_toc_estimate_keys(&estimator, 1); |
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shm_toc_estimate_chunk(&estimator, SESSION_DSA_SIZE); |
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/* Estimate space for the per-session record typmod registry. */ |
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typmod_registry_size = SharedRecordTypmodRegistryEstimate(); |
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shm_toc_estimate_keys(&estimator, 1); |
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shm_toc_estimate_chunk(&estimator, typmod_registry_size); |
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/* Set up segment and TOC. */ |
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size = shm_toc_estimate(&estimator); |
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seg = dsm_create(size, DSM_CREATE_NULL_IF_MAXSEGMENTS); |
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if (seg == NULL) |
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{ |
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MemoryContextSwitchTo(old_context); |
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return DSM_HANDLE_INVALID; |
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} |
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toc = shm_toc_create(SESSION_MAGIC, |
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dsm_segment_address(seg), |
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size); |
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/* Create per-session DSA area. */ |
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dsa_space = shm_toc_allocate(toc, SESSION_DSA_SIZE); |
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dsa = dsa_create_in_place(dsa_space, |
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SESSION_DSA_SIZE, |
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LWTRANCHE_SESSION_DSA, |
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seg); |
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shm_toc_insert(toc, SESSION_KEY_DSA, dsa_space); |
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/* Create session-scoped shared record typmod registry. */ |
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typmod_registry_space = shm_toc_allocate(toc, typmod_registry_size); |
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SharedRecordTypmodRegistryInit((SharedRecordTypmodRegistry *) |
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typmod_registry_space, seg, dsa); |
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shm_toc_insert(toc, SESSION_KEY_RECORD_TYPMOD_REGISTRY, |
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typmod_registry_space); |
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/*
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* If we got this far, we can pin the shared memory so it stays mapped for |
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* the rest of this backend's life. If we don't make it this far, cleanup |
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* callbacks for anything we installed above (ie currently |
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* SharedRecordTypemodRegistry) will run when the DSM segment is detached |
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* by CurrentResourceOwner so we aren't left with a broken CurrentSession. |
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*/ |
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dsm_pin_mapping(seg); |
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dsa_pin_mapping(dsa); |
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/* Make segment and area available via CurrentSession. */ |
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CurrentSession->segment = seg; |
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CurrentSession->area = dsa; |
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MemoryContextSwitchTo(old_context); |
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return dsm_segment_handle(seg); |
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} |
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/*
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* Attach to a per-session DSM segment provided by a parallel leader. |
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*/ |
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void |
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AttachSession(dsm_handle handle) |
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{ |
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dsm_segment *seg; |
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shm_toc *toc; |
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void *dsa_space; |
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void *typmod_registry_space; |
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dsa_area *dsa; |
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MemoryContext old_context; |
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old_context = MemoryContextSwitchTo(TopMemoryContext); |
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/* Attach to the DSM segment. */ |
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seg = dsm_attach(handle); |
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if (seg == NULL) |
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elog(ERROR, "could not attach to per-session DSM segment"); |
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toc = shm_toc_attach(SESSION_MAGIC, dsm_segment_address(seg)); |
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/* Attach to the DSA area. */ |
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dsa_space = shm_toc_lookup(toc, SESSION_KEY_DSA, false); |
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dsa = dsa_attach_in_place(dsa_space, seg); |
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/* Make them available via the current session. */ |
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CurrentSession->segment = seg; |
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CurrentSession->area = dsa; |
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/* Attach to the shared record typmod registry. */ |
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typmod_registry_space = |
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shm_toc_lookup(toc, SESSION_KEY_RECORD_TYPMOD_REGISTRY, false); |
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SharedRecordTypmodRegistryAttach((SharedRecordTypmodRegistry *) |
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typmod_registry_space); |
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/* Remain attached until end of backend or DetachSession(). */ |
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dsm_pin_mapping(seg); |
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dsa_pin_mapping(dsa); |
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MemoryContextSwitchTo(old_context); |
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} |
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/*
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* Detach from the current session DSM segment. It's not strictly necessary |
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* to do this explicitly since we'll detach automatically at backend exit, but |
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* if we ever reuse parallel workers it will become important for workers to |
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* detach from one session before attaching to another. Note that this runs |
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* detach hooks. |
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*/ |
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void |
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DetachSession(void) |
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{ |
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/* Runs detach hooks. */ |
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dsm_detach(CurrentSession->segment); |
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CurrentSession->segment = NULL; |
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dsa_detach(CurrentSession->area); |
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CurrentSession->area = NULL; |
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} |
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@ -0,0 +1,44 @@ |
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/*-------------------------------------------------------------------------
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* |
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* session.h |
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* Encapsulation of user session. |
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* |
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* Copyright (c) 2017, PostgreSQL Global Development Group |
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* |
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* src/include/access/session.h |
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* |
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*------------------------------------------------------------------------- |
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*/ |
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#ifndef SESSION_H |
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#define SESSION_H |
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#include "lib/dshash.h" |
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/* Defined in typcache.c */ |
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typedef struct SharedRecordTypmodRegistry SharedRecordTypmodRegistry; |
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/*
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* A struct encapsulating some elements of a user's session. For now this |
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* manages state that applies to parallel query, but it principle it could |
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* include other things that are currently global variables. |
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*/ |
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typedef struct Session |
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{ |
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dsm_segment *segment; /* The session-scoped DSM segment. */ |
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dsa_area *area; /* The session-scoped DSA area. */ |
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/* State managed by typcache.c. */ |
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SharedRecordTypmodRegistry *shared_typmod_registry; |
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dshash_table *shared_record_table; |
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dshash_table *shared_typmod_table; |
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} Session; |
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extern void InitializeSession(void); |
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extern dsm_handle GetSessionDsmHandle(void); |
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extern void AttachSession(dsm_handle handle); |
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extern void DetachSession(void); |
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/* The current session, or NULL for none. */ |
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extern Session *CurrentSession; |
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#endif /* SESSION_H */ |
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