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521 lines
17 KiB
C
521 lines
17 KiB
C
/*-------------------------------------------------------------------------
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*
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* ts_typanalyze.c
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* functions for gathering statistics from tsvector columns
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*
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* Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
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*
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*
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* IDENTIFICATION
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* src/backend/tsearch/ts_typanalyze.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/hash.h"
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#include "catalog/pg_operator.h"
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#include "commands/vacuum.h"
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#include "tsearch/ts_type.h"
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#include "utils/builtins.h"
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/* A hash key for lexemes */
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typedef struct
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{
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char *lexeme; /* lexeme (not NULL terminated!) */
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int length; /* its length in bytes */
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} LexemeHashKey;
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/* A hash table entry for the Lossy Counting algorithm */
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typedef struct
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{
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LexemeHashKey key; /* This is 'e' from the LC algorithm. */
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int frequency; /* This is 'f'. */
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int delta; /* And this is 'delta'. */
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} TrackItem;
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static void compute_tsvector_stats(VacAttrStats *stats,
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AnalyzeAttrFetchFunc fetchfunc,
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int samplerows,
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double totalrows);
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static void prune_lexemes_hashtable(HTAB *lexemes_tab, int b_current);
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static uint32 lexeme_hash(const void *key, Size keysize);
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static int lexeme_match(const void *key1, const void *key2, Size keysize);
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static int lexeme_compare(const void *key1, const void *key2);
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static int trackitem_compare_frequencies_desc(const void *e1, const void *e2);
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static int trackitem_compare_lexemes(const void *e1, const void *e2);
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/*
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* ts_typanalyze -- a custom typanalyze function for tsvector columns
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*/
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Datum
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ts_typanalyze(PG_FUNCTION_ARGS)
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{
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VacAttrStats *stats = (VacAttrStats *) PG_GETARG_POINTER(0);
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Form_pg_attribute attr = stats->attr;
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/* If the attstattarget column is negative, use the default value */
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/* NB: it is okay to scribble on stats->attr since it's a copy */
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if (attr->attstattarget < 0)
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attr->attstattarget = default_statistics_target;
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stats->compute_stats = compute_tsvector_stats;
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/* see comment about the choice of minrows in commands/analyze.c */
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stats->minrows = 300 * attr->attstattarget;
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PG_RETURN_BOOL(true);
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}
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/*
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* compute_tsvector_stats() -- compute statistics for a tsvector column
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*
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* This functions computes statistics that are useful for determining @@
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* operations' selectivity, along with the fraction of non-null rows and
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* average width.
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*
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* Instead of finding the most common values, as we do for most datatypes,
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* we're looking for the most common lexemes. This is more useful, because
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* there most probably won't be any two rows with the same tsvector and thus
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* the notion of a MCV is a bit bogus with this datatype. With a list of the
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* most common lexemes we can do a better job at figuring out @@ selectivity.
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*
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* For the same reasons we assume that tsvector columns are unique when
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* determining the number of distinct values.
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*
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* The algorithm used is Lossy Counting, as proposed in the paper "Approximate
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* frequency counts over data streams" by G. S. Manku and R. Motwani, in
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* Proceedings of the 28th International Conference on Very Large Data Bases,
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* Hong Kong, China, August 2002, section 4.2. The paper is available at
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* http://www.vldb.org/conf/2002/S10P03.pdf
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*
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* The Lossy Counting (aka LC) algorithm goes like this:
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* Let s be the threshold frequency for an item (the minimum frequency we
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* are interested in) and epsilon the error margin for the frequency. Let D
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* be a set of triples (e, f, delta), where e is an element value, f is that
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* element's frequency (actually, its current occurrence count) and delta is
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* the maximum error in f. We start with D empty and process the elements in
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* batches of size w. (The batch size is also known as "bucket size" and is
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* equal to 1/epsilon.) Let the current batch number be b_current, starting
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* with 1. For each element e we either increment its f count, if it's
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* already in D, or insert a new triple into D with values (e, 1, b_current
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* - 1). After processing each batch we prune D, by removing from it all
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* elements with f + delta <= b_current. After the algorithm finishes we
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* suppress all elements from D that do not satisfy f >= (s - epsilon) * N,
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* where N is the total number of elements in the input. We emit the
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* remaining elements with estimated frequency f/N. The LC paper proves
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* that this algorithm finds all elements with true frequency at least s,
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* and that no frequency is overestimated or is underestimated by more than
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* epsilon. Furthermore, given reasonable assumptions about the input
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* distribution, the required table size is no more than about 7 times w.
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*
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* We set s to be the estimated frequency of the K'th word in a natural
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* language's frequency table, where K is the target number of entries in
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* the MCELEM array plus an arbitrary constant, meant to reflect the fact
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* that the most common words in any language would usually be stopwords
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* so we will not actually see them in the input. We assume that the
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* distribution of word frequencies (including the stopwords) follows Zipf's
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* law with an exponent of 1.
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*
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* Assuming Zipfian distribution, the frequency of the K'th word is equal
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* to 1/(K * H(W)) where H(n) is 1/2 + 1/3 + ... + 1/n and W is the number of
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* words in the language. Putting W as one million, we get roughly 0.07/K.
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* Assuming top 10 words are stopwords gives s = 0.07/(K + 10). We set
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* epsilon = s/10, which gives bucket width w = (K + 10)/0.007 and
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* maximum expected hashtable size of about 1000 * (K + 10).
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*
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* Note: in the above discussion, s, epsilon, and f/N are in terms of a
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* lexeme's frequency as a fraction of all lexemes seen in the input.
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* However, what we actually want to store in the finished pg_statistic
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* entry is each lexeme's frequency as a fraction of all rows that it occurs
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* in. Assuming that the input tsvectors are correctly constructed, no
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* lexeme occurs more than once per tsvector, so the final count f is a
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* correct estimate of the number of input tsvectors it occurs in, and we
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* need only change the divisor from N to nonnull_cnt to get the number we
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* want.
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*/
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static void
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compute_tsvector_stats(VacAttrStats *stats,
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AnalyzeAttrFetchFunc fetchfunc,
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int samplerows,
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double totalrows)
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{
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int num_mcelem;
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int null_cnt = 0;
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double total_width = 0;
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/* This is D from the LC algorithm. */
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HTAB *lexemes_tab;
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HASHCTL hash_ctl;
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HASH_SEQ_STATUS scan_status;
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/* This is the current bucket number from the LC algorithm */
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int b_current;
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/* This is 'w' from the LC algorithm */
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int bucket_width;
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int vector_no,
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lexeme_no;
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LexemeHashKey hash_key;
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TrackItem *item;
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/*
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* We want statistics_target * 10 lexemes in the MCELEM array. This
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* multiplier is pretty arbitrary, but is meant to reflect the fact that
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* the number of individual lexeme values tracked in pg_statistic ought to
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* be more than the number of values for a simple scalar column.
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*/
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num_mcelem = stats->attr->attstattarget * 10;
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/*
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* We set bucket width equal to (num_mcelem + 10) / 0.007 as per the
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* comment above.
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*/
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bucket_width = (num_mcelem + 10) * 1000 / 7;
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/*
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* Create the hashtable. It will be in local memory, so we don't need to
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* worry about overflowing the initial size. Also we don't need to pay any
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* attention to locking and memory management.
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*/
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MemSet(&hash_ctl, 0, sizeof(hash_ctl));
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hash_ctl.keysize = sizeof(LexemeHashKey);
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hash_ctl.entrysize = sizeof(TrackItem);
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hash_ctl.hash = lexeme_hash;
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hash_ctl.match = lexeme_match;
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hash_ctl.hcxt = CurrentMemoryContext;
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lexemes_tab = hash_create("Analyzed lexemes table",
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num_mcelem,
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&hash_ctl,
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HASH_ELEM | HASH_FUNCTION | HASH_COMPARE | HASH_CONTEXT);
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/* Initialize counters. */
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b_current = 1;
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lexeme_no = 0;
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/* Loop over the tsvectors. */
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for (vector_no = 0; vector_no < samplerows; vector_no++)
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{
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Datum value;
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bool isnull;
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TSVector vector;
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WordEntry *curentryptr;
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char *lexemesptr;
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int j;
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vacuum_delay_point();
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value = fetchfunc(stats, vector_no, &isnull);
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/*
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* Check for null/nonnull.
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*/
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if (isnull)
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{
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null_cnt++;
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continue;
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}
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/*
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* Add up widths for average-width calculation. Since it's a
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* tsvector, we know it's varlena. As in the regular
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* compute_minimal_stats function, we use the toasted width for this
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* calculation.
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*/
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total_width += VARSIZE_ANY(DatumGetPointer(value));
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/*
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* Now detoast the tsvector if needed.
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*/
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vector = DatumGetTSVector(value);
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/*
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* We loop through the lexemes in the tsvector and add them to our
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* tracking hashtable. Note: the hashtable entries will point into
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* the (detoasted) tsvector value, therefore we cannot free that
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* storage until we're done.
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*/
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lexemesptr = STRPTR(vector);
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curentryptr = ARRPTR(vector);
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for (j = 0; j < vector->size; j++)
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{
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bool found;
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/* Construct a hash key */
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hash_key.lexeme = lexemesptr + curentryptr->pos;
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hash_key.length = curentryptr->len;
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/* Lookup current lexeme in hashtable, adding it if new */
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item = (TrackItem *) hash_search(lexemes_tab,
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(const void *) &hash_key,
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HASH_ENTER, &found);
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if (found)
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{
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/* The lexeme is already on the tracking list */
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item->frequency++;
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}
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else
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{
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/* Initialize new tracking list element */
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item->frequency = 1;
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item->delta = b_current - 1;
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}
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/* lexeme_no is the number of elements processed (ie N) */
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lexeme_no++;
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/* We prune the D structure after processing each bucket */
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if (lexeme_no % bucket_width == 0)
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{
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prune_lexemes_hashtable(lexemes_tab, b_current);
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b_current++;
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}
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/* Advance to the next WordEntry in the tsvector */
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curentryptr++;
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}
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}
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/* We can only compute real stats if we found some non-null values. */
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if (null_cnt < samplerows)
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{
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int nonnull_cnt = samplerows - null_cnt;
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int i;
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TrackItem **sort_table;
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int track_len;
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int cutoff_freq;
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int minfreq,
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maxfreq;
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stats->stats_valid = true;
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/* Do the simple null-frac and average width stats */
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stats->stanullfrac = (double) null_cnt / (double) samplerows;
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stats->stawidth = total_width / (double) nonnull_cnt;
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/* Assume it's a unique column (see notes above) */
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stats->stadistinct = -1.0 * (1.0 - stats->stanullfrac);
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/*
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* Construct an array of the interesting hashtable items, that is,
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* those meeting the cutoff frequency (s - epsilon)*N. Also identify
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* the minimum and maximum frequencies among these items.
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*
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* Since epsilon = s/10 and bucket_width = 1/epsilon, the cutoff
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* frequency is 9*N / bucket_width.
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*/
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cutoff_freq = 9 * lexeme_no / bucket_width;
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i = hash_get_num_entries(lexemes_tab); /* surely enough space */
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sort_table = (TrackItem **) palloc(sizeof(TrackItem *) * i);
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hash_seq_init(&scan_status, lexemes_tab);
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track_len = 0;
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minfreq = lexeme_no;
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maxfreq = 0;
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while ((item = (TrackItem *) hash_seq_search(&scan_status)) != NULL)
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{
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if (item->frequency > cutoff_freq)
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{
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sort_table[track_len++] = item;
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minfreq = Min(minfreq, item->frequency);
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maxfreq = Max(maxfreq, item->frequency);
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}
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}
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Assert(track_len <= i);
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/* emit some statistics for debug purposes */
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elog(DEBUG3, "tsvector_stats: target # mces = %d, bucket width = %d, "
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"# lexemes = %d, hashtable size = %d, usable entries = %d",
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num_mcelem, bucket_width, lexeme_no, i, track_len);
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/*
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* If we obtained more lexemes than we really want, get rid of those
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* with least frequencies. The easiest way is to qsort the array into
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* descending frequency order and truncate the array.
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*/
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if (num_mcelem < track_len)
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{
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qsort(sort_table, track_len, sizeof(TrackItem *),
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trackitem_compare_frequencies_desc);
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/* reset minfreq to the smallest frequency we're keeping */
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minfreq = sort_table[num_mcelem - 1]->frequency;
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}
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else
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num_mcelem = track_len;
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/* Generate MCELEM slot entry */
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if (num_mcelem > 0)
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{
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MemoryContext old_context;
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Datum *mcelem_values;
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float4 *mcelem_freqs;
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/*
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* We want to store statistics sorted on the lexeme value using
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* first length, then byte-for-byte comparison. The reason for
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* doing length comparison first is that we don't care about the
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* ordering so long as it's consistent, and comparing lengths
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* first gives us a chance to avoid a strncmp() call.
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*
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* This is different from what we do with scalar statistics --
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* they get sorted on frequencies. The rationale is that we
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* usually search through most common elements looking for a
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* specific value, so we can grab its frequency. When values are
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* presorted we can employ binary search for that. See
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* ts_selfuncs.c for a real usage scenario.
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*/
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qsort(sort_table, num_mcelem, sizeof(TrackItem *),
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trackitem_compare_lexemes);
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/* Must copy the target values into anl_context */
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old_context = MemoryContextSwitchTo(stats->anl_context);
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/*
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* We sorted statistics on the lexeme value, but we want to be
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* able to find out the minimal and maximal frequency without
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* going through all the values. We keep those two extra
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* frequencies in two extra cells in mcelem_freqs.
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*
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* (Note: the MCELEM statistics slot definition allows for a third
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* extra number containing the frequency of nulls, but we don't
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* create that for a tsvector column, since null elements aren't
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* possible.)
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*/
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mcelem_values = (Datum *) palloc(num_mcelem * sizeof(Datum));
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mcelem_freqs = (float4 *) palloc((num_mcelem + 2) * sizeof(float4));
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/*
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* See comments above about use of nonnull_cnt as the divisor for
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* the final frequency estimates.
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*/
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for (i = 0; i < num_mcelem; i++)
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{
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TrackItem *item = sort_table[i];
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mcelem_values[i] =
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PointerGetDatum(cstring_to_text_with_len(item->key.lexeme,
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item->key.length));
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mcelem_freqs[i] = (double) item->frequency / (double) nonnull_cnt;
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}
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mcelem_freqs[i++] = (double) minfreq / (double) nonnull_cnt;
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mcelem_freqs[i] = (double) maxfreq / (double) nonnull_cnt;
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MemoryContextSwitchTo(old_context);
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stats->stakind[0] = STATISTIC_KIND_MCELEM;
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stats->staop[0] = TextEqualOperator;
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stats->stanumbers[0] = mcelem_freqs;
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/* See above comment about two extra frequency fields */
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stats->numnumbers[0] = num_mcelem + 2;
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stats->stavalues[0] = mcelem_values;
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stats->numvalues[0] = num_mcelem;
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/* We are storing text values */
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stats->statypid[0] = TEXTOID;
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stats->statyplen[0] = -1; /* typlen, -1 for varlena */
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stats->statypbyval[0] = false;
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stats->statypalign[0] = 'i';
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}
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}
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else
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{
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/* We found only nulls; assume the column is entirely null */
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stats->stats_valid = true;
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stats->stanullfrac = 1.0;
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stats->stawidth = 0; /* "unknown" */
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stats->stadistinct = 0.0; /* "unknown" */
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}
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/*
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* We don't need to bother cleaning up any of our temporary palloc's. The
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* hashtable should also go away, as it used a child memory context.
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*/
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}
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/*
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* A function to prune the D structure from the Lossy Counting algorithm.
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* Consult compute_tsvector_stats() for wider explanation.
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*/
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static void
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prune_lexemes_hashtable(HTAB *lexemes_tab, int b_current)
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{
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HASH_SEQ_STATUS scan_status;
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TrackItem *item;
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hash_seq_init(&scan_status, lexemes_tab);
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while ((item = (TrackItem *) hash_seq_search(&scan_status)) != NULL)
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{
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if (item->frequency + item->delta <= b_current)
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{
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if (hash_search(lexemes_tab, (const void *) &item->key,
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HASH_REMOVE, NULL) == NULL)
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elog(ERROR, "hash table corrupted");
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}
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}
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}
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/*
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* Hash functions for lexemes. They are strings, but not NULL terminated,
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* so we need a special hash function.
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*/
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static uint32
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lexeme_hash(const void *key, Size keysize)
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{
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const LexemeHashKey *l = (const LexemeHashKey *) key;
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return DatumGetUInt32(hash_any((const unsigned char *) l->lexeme,
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l->length));
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}
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/*
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* Matching function for lexemes, to be used in hashtable lookups.
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*/
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static int
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lexeme_match(const void *key1, const void *key2, Size keysize)
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{
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/* The keysize parameter is superfluous, the keys store their lengths */
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return lexeme_compare(key1, key2);
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}
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/*
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* Comparison function for lexemes.
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*/
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static int
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lexeme_compare(const void *key1, const void *key2)
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{
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const LexemeHashKey *d1 = (const LexemeHashKey *) key1;
|
|
const LexemeHashKey *d2 = (const LexemeHashKey *) key2;
|
|
|
|
/* First, compare by length */
|
|
if (d1->length > d2->length)
|
|
return 1;
|
|
else if (d1->length < d2->length)
|
|
return -1;
|
|
/* Lengths are equal, do a byte-by-byte comparison */
|
|
return strncmp(d1->lexeme, d2->lexeme, d1->length);
|
|
}
|
|
|
|
/*
|
|
* qsort() comparator for sorting TrackItems on frequencies (descending sort)
|
|
*/
|
|
static int
|
|
trackitem_compare_frequencies_desc(const void *e1, const void *e2)
|
|
{
|
|
const TrackItem *const * t1 = (const TrackItem *const *) e1;
|
|
const TrackItem *const * t2 = (const TrackItem *const *) e2;
|
|
|
|
return (*t2)->frequency - (*t1)->frequency;
|
|
}
|
|
|
|
/*
|
|
* qsort() comparator for sorting TrackItems on lexemes
|
|
*/
|
|
static int
|
|
trackitem_compare_lexemes(const void *e1, const void *e2)
|
|
{
|
|
const TrackItem *const * t1 = (const TrackItem *const *) e1;
|
|
const TrackItem *const * t2 = (const TrackItem *const *) e2;
|
|
|
|
return lexeme_compare(&(*t1)->key, &(*t2)->key);
|
|
}
|