Re: index access method documentation light on details on ii_AmCache
От | Euler Taveira |
---|---|
Тема | Re: index access method documentation light on details on ii_AmCache |
Дата | |
Msg-id | a52489dd-e6ca-425b-a23d-d0139728419f@app.fastmail.com обсуждение исходный текст |
Ответ на | index access method documentation light on details on ii_AmCache (PG Doc comments form <noreply@postgresql.org>) |
Ответы |
Re: index access method documentation light on details on ii_AmCache
|
Список | pgsql-docs |
On Tue, Sep 12, 2023, at 2:36 PM, PG Doc comments form wrote:
The following documentation comment has been logged on the website:Description:So, if I cache something in ii_AmCache during a call to my aminsertcallback...When, if ever, does it get freed?Having looked at example code, I don't actually see anything doing this ininsert paths, so presumably there's some point at which this happensautomatically, possibly as part of the Memory Context thing, maybe relatedto the ii_Context which seems to be getting used, but I can't find anythinganywhere documenting that. This may well be completely obvious, or intendedto be implied by "it can allocate space in indexInfo->ii_Context", but it'snot exceptionally obvious to me as a newcomer to the code. (By contrast, theambuild docs say to palloc a data structure, but don't mention a context forit; no idea whether it should be in a particular context.)
Maybe it is not clear by the comment in the IndexInfo struct but ii_AmCache is
a pointer to the AM state information that is stored into ii_Context memory
context (see gistinsert, gininsert or brininsert to understand how ii_Context
and ii_AmCache are used). The document that you shared also has a sentence with
this information.
Unless you change it, ii_Context is CurrentMemoryContext (see makeIndexInfo).
Hence, your AM state information is freed when the current memory context is
freed. You can use gdb to figure out what's the memory context you are in.
Start a session, create a table and an index on it. Attach gdb to this current
session and add a breakpoint into one of the AM insert function that I
mentioned in the previous paragraph. Insert a row and continue:
(gdb) b initGinState
Breakpoint 1 at 0x55a47b179d80: file ginutil.c, line 98.
(gdb) c
Continuing.
Breakpoint 1, initGinState (state=state@entry=0x55a47d40cd58, index=index@entry=0x7f1970352d40)
at ginutil.c:98
(gdb) bt
#0 initGinState (state=state@entry=0x55a47d40cd58, index=index@entry=0x7f1970352d40) at ginutil.c:98
#1 0x000055a47b178255 in gininsert (index=0x7f1970352d40, values=0x7fff31d04a80, isnull=0x7fff31d04a60,
ht_ctid=0x55a47d415ef8, heapRel=<optimized out>, checkUnique=<optimized out>, indexUnchanged=false,
indexInfo=0x55a47d4164f8) at gininsert.c:502
#2 0x000055a47b3100f6 in ExecInsertIndexTuples (resultRelInfo=resultRelInfo@entry=0x55a47d415328,
slot=slot@entry=0x55a47d415ec8, estate=estate@entry=0x55a47d414e98, update=update@entry=false,
noDupErr=noDupErr@entry=false, specConflict=specConflict@entry=0x0, arbiterIndexes=0x0,
onlySummarizing=false) at execIndexing.c:432
.
.
.
(gdb) f 1
#1 0x000055a47b178255 in gininsert (index=0x7f1970352d40, values=0x7fff31d04a80, isnull=0x7fff31d04a60,
ht_ctid=0x55a47d415ef8, heapRel=<optimized out>, checkUnique=<optimized out>, indexUnchanged=false,
indexInfo=0x55a47d4164f8) at gininsert.c:502
(gdb) p *indexInfo->ii_Context
$3 = {type = T_AllocSetContext, isReset = false, allowInCritSection = false, mem_allocated = 17912,
methods = 0x55a47b9302f0 <mcxt_methods+240>, parent = 0x55a47d3fdda0, firstchild = 0x55a47d408cd0,
prevchild = 0x0, nextchild = 0x0, name = 0x55a47b751813 "ExecutorState", ident = 0x0, reset_cbs = 0x0}
The "ExecutorState" is a per query memory context so it is deallocated when the
query ends.
Actually, in full generality, I have not been able to find a section of thedocumentation which explains the memory-context stuff at all. I found a blogpost elsewhere suggesting that it's just "the memory context will be freedand thus everything associated with it". This implies that there's nostraightforward way for an index to do end-of-insert maintenance after allthe inserts from a given query are complete, except to do it after everytuple just in case it's the last tuple, I guess?
Check src/backend/utils/mmgr/README.
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