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Commit21025d4

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Use a hash table to store current sequence values.
This speeds up nextval() and currval(), when you touch a lot of differentsequences in the same backend.David Rowley
1 parent982b82d commit21025d4

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1 file changed

+35
-36
lines changed

1 file changed

+35
-36
lines changed

‎src/backend/commands/sequence.c

Lines changed: 35 additions & 36 deletions
Original file line numberDiff line numberDiff line change
@@ -60,15 +60,10 @@ typedef struct sequence_magic
6060
* session. This is needed to hold onto nextval/currval state. (We can't
6161
* rely on the relcache, since it's only, well, a cache, and may decide to
6262
* discard entries.)
63-
*
64-
* XXX We use linear search to find pre-existing SeqTable entries.This is
65-
* good when only a small number of sequences are touched in a session, but
66-
* would suck with many different sequences. Perhaps use a hashtable someday.
6763
*/
6864
typedefstructSeqTableData
6965
{
70-
structSeqTableData*next;/* link to next SeqTable object */
71-
Oidrelid;/* pg_class OID of this sequence */
66+
Oidrelid;/* pg_class OID of this sequence (hash key) */
7267
Oidfilenode;/* last seen relfilenode of this sequence */
7368
LocalTransactionIdlxid;/* xact in which we last did a seq op */
7469
boollast_valid;/* do we have a valid "last" value? */
@@ -81,7 +76,7 @@ typedef struct SeqTableData
8176

8277
typedefSeqTableData*SeqTable;
8378

84-
staticSeqTableseqtab=NULL;/*Head of list of SeqTable items */
79+
staticHTAB*seqhashtab=NULL;/*hash table for SeqTable items */
8580

8681
/*
8782
* last_used_seq is updated by nextval() to point to the last used
@@ -92,6 +87,7 @@ static SeqTableData *last_used_seq = NULL;
9287
staticvoidfill_seq_with_data(Relationrel,HeapTupletuple);
9388
staticint64nextval_internal(Oidrelid);
9489
staticRelationopen_share_lock(SeqTableseq);
90+
staticvoidcreate_seq_hashtable(void);
9591
staticvoidinit_sequence(Oidrelid,SeqTable*p_elm,Relation*p_rel);
9692
staticForm_pg_sequenceread_seq_tuple(SeqTableelm,Relationrel,
9793
Buffer*buf,HeapTupleseqtuple);
@@ -998,6 +994,23 @@ open_share_lock(SeqTable seq)
998994
returnrelation_open(seq->relid,NoLock);
999995
}
1000996

997+
/*
998+
* Creates the hash table for storing sequence data
999+
*/
1000+
staticvoid
1001+
create_seq_hashtable(void)
1002+
{
1003+
HASHCTLctl;
1004+
1005+
memset(&ctl,0,sizeof(ctl));
1006+
ctl.keysize=sizeof(Oid);
1007+
ctl.entrysize=sizeof(SeqTableData);
1008+
ctl.hash=oid_hash;
1009+
1010+
seqhashtab=hash_create("Sequence values",16,&ctl,
1011+
HASH_ELEM |HASH_FUNCTION |HASH_CONTEXT);
1012+
}
1013+
10011014
/*
10021015
* Given a relation OID, open and lock the sequence. p_elm and p_rel are
10031016
* output parameters.
@@ -1007,39 +1020,28 @@ init_sequence(Oid relid, SeqTable *p_elm, Relation *p_rel)
10071020
{
10081021
SeqTableelm;
10091022
Relationseqrel;
1023+
boolfound;
10101024

1011-
/* Look to see if we already have a seqtable entry for relation */
1012-
for (elm=seqtab;elm!=NULL;elm=elm->next)
1013-
{
1014-
if (elm->relid==relid)
1015-
break;
1016-
}
1025+
if (seqhashtab==NULL)
1026+
create_seq_hashtable();
1027+
1028+
elm= (SeqTable)hash_search(seqhashtab,&relid,HASH_ENTER,&found);
10171029

10181030
/*
1019-
*Allocatenewseqtableentry ifwe didn't find one.
1031+
*Initalize thenewhash tableentry ifit did not exist already.
10201032
*
1021-
* NOTE: seqtable entries remain in the list for the life of a backend. If
1022-
* the sequence itself is deleted then the entry becomes wasted memory,
1023-
* but it's small enough that this should not matter.
1033+
* NOTE: seqtable entries are stored for the life of a backend (unless
1034+
* explictly discarded with DISCARD). If the sequence itself is deleted
1035+
* then the entry becomes wasted memory, but it's small enough that this
1036+
* should not matter.
10241037
*/
1025-
if (elm==NULL)
1038+
if (!found)
10261039
{
1027-
/*
1028-
* Time to make a new seqtable entry. These entries live as long as
1029-
* the backend does, so we use plain malloc for them.
1030-
*/
1031-
elm= (SeqTable)malloc(sizeof(SeqTableData));
1032-
if (elm==NULL)
1033-
ereport(ERROR,
1034-
(errcode(ERRCODE_OUT_OF_MEMORY),
1035-
errmsg("out of memory")));
1036-
elm->relid=relid;
1040+
/* relid already filled in */
10371041
elm->filenode=InvalidOid;
10381042
elm->lxid=InvalidLocalTransactionId;
10391043
elm->last_valid= false;
10401044
elm->last=elm->cached=elm->increment=0;
1041-
elm->next=seqtab;
1042-
seqtab=elm;
10431045
}
10441046

10451047
/*
@@ -1609,13 +1611,10 @@ seq_redo(XLogRecPtr lsn, XLogRecord *record)
16091611
void
16101612
ResetSequenceCaches(void)
16111613
{
1612-
SeqTableData*next;
1613-
1614-
while (seqtab!=NULL)
1614+
if (seqhashtab)
16151615
{
1616-
next=seqtab->next;
1617-
free(seqtab);
1618-
seqtab=next;
1616+
hash_destroy(seqhashtab);
1617+
seqhashtab=NULL;
16191618
}
16201619

16211620
last_used_seq=NULL;

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