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TABLESAMPLE, SQL Standard and extensible

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Add a TABLESAMPLE clause to SELECT statements that allows
user to specify random BERNOULLI sampling or block level
SYSTEM sampling. Implementation allows for extensible
sampling functions to be written, using a standard API.
Basic version follows SQLStandard exactly. Usable
concrete use cases for the sampling API follow in later
commits.

Petr Jelinek

Reviewed by Michael Paquier and Simon Riggs
This commit is contained in:
Simon Riggs
2015-05-15 14:37:10 -04:00
parent 11a83bbedd
commit f6d208d6e5
66 changed files with 2756 additions and 40 deletions
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41
src/backend/access/heap/heapam.c
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@ -80,8 +80,9 @@ bool synchronize_seqscans = true;
static HeapScanDesc heap_beginscan_internal(Relation relation,
Snapshot snapshot,
int nkeys, ScanKey key,
bool allow_strat, bool allow_sync,
bool is_bitmapscan, bool temp_snap);
bool allow_strat, bool allow_sync, bool allow_pagemode,
bool is_bitmapscan, bool is_samplescan,
bool temp_snap);
static HeapTuple heap_prepare_insert(Relation relation, HeapTuple tup,
TransactionId xid, CommandId cid, int options);
static XLogRecPtr log_heap_update(Relation reln, Buffer oldbuf,
@ -294,9 +295,10 @@ initscan(HeapScanDesc scan, ScanKey key, bool is_rescan)
/*
* Currently, we don't have a stats counter for bitmap heap scans (but the
* underlying bitmap index scans will be counted).
* underlying bitmap index scans will be counted) or sample scans (we only
* update stats for tuple fetches there)
*/
if (!scan->rs_bitmapscan)
if (!scan->rs_bitmapscan && !scan->rs_samplescan)
pgstat_count_heap_scan(scan->rs_rd);
}
@ -315,7 +317,7 @@ heap_setscanlimits(HeapScanDesc scan, BlockNumber startBlk, BlockNumber numBlks)
* In page-at-a-time mode it performs additional work, namely determining
* which tuples on the page are visible.
*/
static void
void
heapgetpage(HeapScanDesc scan, BlockNumber page)
{
Buffer buffer;
@ -1310,6 +1312,9 @@ heap_openrv_extended(const RangeVar *relation, LOCKMODE lockmode,
* HeapScanDesc for a bitmap heap scan. Although that scan technology is
* really quite unlike a standard seqscan, there is just enough commonality
* to make it worth using the same data structure.
*
* heap_beginscan_samplingscan is alternate entry point for setting up a
* HeapScanDesc for a TABLESAMPLE scan.
* ----------------
*/
HeapScanDesc
@ -1317,7 +1322,7 @@ heap_beginscan(Relation relation, Snapshot snapshot,
int nkeys, ScanKey key)
{
return heap_beginscan_internal(relation, snapshot, nkeys, key,
true, true, false, false);
true, true, true, false, false, false);
}
HeapScanDesc
@ -1327,7 +1332,7 @@ heap_beginscan_catalog(Relation relation, int nkeys, ScanKey key)
Snapshot snapshot = RegisterSnapshot(GetCatalogSnapshot(relid));
return heap_beginscan_internal(relation, snapshot, nkeys, key,
true, true, false, true);
true, true, true, false, false, true);
}
HeapScanDesc
@ -1336,7 +1341,8 @@ heap_beginscan_strat(Relation relation, Snapshot snapshot,
bool allow_strat, bool allow_sync)
{
return heap_beginscan_internal(relation, snapshot, nkeys, key,
allow_strat, allow_sync, false, false);
allow_strat, allow_sync, true,
false, false, false);
}
HeapScanDesc
@ -1344,14 +1350,24 @@ heap_beginscan_bm(Relation relation, Snapshot snapshot,
int nkeys, ScanKey key)
{
return heap_beginscan_internal(relation, snapshot, nkeys, key,
false, false, true, false);
false, false, true, true, false, false);
}
HeapScanDesc
heap_beginscan_sampling(Relation relation, Snapshot snapshot,
int nkeys, ScanKey key,
bool allow_strat, bool allow_pagemode)
{
return heap_beginscan_internal(relation, snapshot, nkeys, key,
allow_strat, false, allow_pagemode,
false, true, false);
}
static HeapScanDesc
heap_beginscan_internal(Relation relation, Snapshot snapshot,
int nkeys, ScanKey key,
bool allow_strat, bool allow_sync,
bool is_bitmapscan, bool temp_snap)
bool allow_strat, bool allow_sync, bool allow_pagemode,
bool is_bitmapscan, bool is_samplescan, bool temp_snap)
{
HeapScanDesc scan;
@ -1373,6 +1389,7 @@ heap_beginscan_internal(Relation relation, Snapshot snapshot,
scan->rs_snapshot = snapshot;
scan->rs_nkeys = nkeys;
scan->rs_bitmapscan = is_bitmapscan;
scan->rs_samplescan = is_samplescan;
scan->rs_strategy = NULL; /* set in initscan */
scan->rs_allow_strat = allow_strat;
scan->rs_allow_sync = allow_sync;
@ -1381,7 +1398,7 @@ heap_beginscan_internal(Relation relation, Snapshot snapshot,
/*
* we can use page-at-a-time mode if it's an MVCC-safe snapshot
*/
scan->rs_pageatatime = IsMVCCSnapshot(snapshot);
scan->rs_pageatatime = allow_pagemode && IsMVCCSnapshot(snapshot);
/*
* For a seqscan in a serializable transaction, acquire a predicate lock