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We can only support a lossy distance function when the distance function's datatype is comparable with the original ordering operator's datatype. The distance function always returns a float8, so we are limited to float8, and float4 (by a hard-coded cast of the float8 to float4). In light of this limitation, it seems like a good idea to have a separate 'recheck' flag for the ORDER BY expressions, so that if you have a non-lossy distance function, it still works with lossy quals. There are cases like that with the build-in or contrib opclasses, but it's plausible. There was a hidden assumption that the ORDER BY values returned by GiST match the original ordering operator's return type, but there are plenty of examples where that's not true, e.g. in btree_gist and pg_trgm. As long as the distance function is not lossy, we can tolerate that and just not return the distance to the executor (or rather, always return NULL). The executor doesn't need the distances if there are no lossy results. There was another little bug: the recheck variable was not initialized before calling the distance function. That revealed the bigger issue, as the executor tried to reorder tuples that didn't need reordering, and that failed because of the datatype mismatch.
119 lines
4.7 KiB
C
119 lines
4.7 KiB
C
/*-------------------------------------------------------------------------
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*
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* relscan.h
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* POSTGRES relation scan descriptor definitions.
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*
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*
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* Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* src/include/access/relscan.h
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*
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*-------------------------------------------------------------------------
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*/
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#ifndef RELSCAN_H
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#define RELSCAN_H
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#include "access/genam.h"
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#include "access/heapam.h"
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#include "access/htup_details.h"
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#include "access/itup.h"
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#include "access/tupdesc.h"
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typedef struct HeapScanDescData
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{
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/* scan parameters */
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Relation rs_rd; /* heap relation descriptor */
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Snapshot rs_snapshot; /* snapshot to see */
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int rs_nkeys; /* number of scan keys */
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ScanKey rs_key; /* array of scan key descriptors */
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bool rs_bitmapscan; /* true if this is really a bitmap scan */
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bool rs_pageatatime; /* verify visibility page-at-a-time? */
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bool rs_allow_strat; /* allow or disallow use of access strategy */
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bool rs_allow_sync; /* allow or disallow use of syncscan */
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bool rs_temp_snap; /* unregister snapshot at scan end? */
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/* state set up at initscan time */
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BlockNumber rs_nblocks; /* total number of blocks in rel */
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BlockNumber rs_startblock; /* block # to start at */
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BlockNumber rs_initblock; /* block # to consider initial of rel */
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BlockNumber rs_numblocks; /* number of blocks to scan */
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BufferAccessStrategy rs_strategy; /* access strategy for reads */
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bool rs_syncscan; /* report location to syncscan logic? */
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/* scan current state */
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bool rs_inited; /* false = scan not init'd yet */
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HeapTupleData rs_ctup; /* current tuple in scan, if any */
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BlockNumber rs_cblock; /* current block # in scan, if any */
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Buffer rs_cbuf; /* current buffer in scan, if any */
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/* NB: if rs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
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/* these fields only used in page-at-a-time mode and for bitmap scans */
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int rs_cindex; /* current tuple's index in vistuples */
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int rs_ntuples; /* number of visible tuples on page */
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OffsetNumber rs_vistuples[MaxHeapTuplesPerPage]; /* their offsets */
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} HeapScanDescData;
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/*
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* We use the same IndexScanDescData structure for both amgettuple-based
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* and amgetbitmap-based index scans. Some fields are only relevant in
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* amgettuple-based scans.
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*/
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typedef struct IndexScanDescData
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{
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/* scan parameters */
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Relation heapRelation; /* heap relation descriptor, or NULL */
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Relation indexRelation; /* index relation descriptor */
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Snapshot xs_snapshot; /* snapshot to see */
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int numberOfKeys; /* number of index qualifier conditions */
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int numberOfOrderBys; /* number of ordering operators */
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ScanKey keyData; /* array of index qualifier descriptors */
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ScanKey orderByData; /* array of ordering op descriptors */
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bool xs_want_itup; /* caller requests index tuples */
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/* signaling to index AM about killing index tuples */
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bool kill_prior_tuple; /* last-returned tuple is dead */
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bool ignore_killed_tuples; /* do not return killed entries */
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bool xactStartedInRecovery; /* prevents killing/seeing killed
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* tuples */
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/* index access method's private state */
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void *opaque; /* access-method-specific info */
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/* in an index-only scan, this is valid after a successful amgettuple */
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IndexTuple xs_itup; /* index tuple returned by AM */
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TupleDesc xs_itupdesc; /* rowtype descriptor of xs_itup */
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/* xs_ctup/xs_cbuf/xs_recheck are valid after a successful index_getnext */
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HeapTupleData xs_ctup; /* current heap tuple, if any */
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Buffer xs_cbuf; /* current heap buffer in scan, if any */
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/* NB: if xs_cbuf is not InvalidBuffer, we hold a pin on that buffer */
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bool xs_recheck; /* T means scan keys must be rechecked */
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/*
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* When fetching with an ordering operator, the values of the ORDER BY
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* expressions of the last returned tuple, according to the index. If
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* xs_recheck is true, these need to be rechecked just like the scan keys,
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* and the values returned here are a lower-bound on the actual values.
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*/
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Datum *xs_orderbyvals;
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bool *xs_orderbynulls;
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bool xs_recheckorderby; /* T means ORDER BY exprs must be rechecked */
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/* state data for traversing HOT chains in index_getnext */
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bool xs_continue_hot; /* T if must keep walking HOT chain */
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} IndexScanDescData;
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/* Struct for heap-or-index scans of system tables */
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typedef struct SysScanDescData
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{
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Relation heap_rel; /* catalog being scanned */
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Relation irel; /* NULL if doing heap scan */
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HeapScanDesc scan; /* only valid in heap-scan case */
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IndexScanDesc iscan; /* only valid in index-scan case */
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Snapshot snapshot; /* snapshot to unregister at end of scan */
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} SysScanDescData;
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#endif /* RELSCAN_H */
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