/*------------------------------------------------------------------------- * * array_typanalyze.c * Functions for gathering statistics from array columns * * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * * * IDENTIFICATION * src/backend/utils/adt/array_typanalyze.c * *------------------------------------------------------------------------- */ #include "postgres.h" #include "access/detoast.h" #include "commands/vacuum.h" #include "utils/array.h" #include "utils/builtins.h" #include "utils/datum.h" #include "utils/lsyscache.h" #include "utils/typcache.h" /* * To avoid consuming too much memory, IO and CPU load during analysis, and/or * too much space in the resulting pg_statistic rows, we ignore arrays that * are wider than ARRAY_WIDTH_THRESHOLD (after detoasting!). Note that this * number is considerably more than the similar WIDTH_THRESHOLD limit used * in analyze.c's standard typanalyze code. */ #define ARRAY_WIDTH_THRESHOLD 0x10000 /* Extra data for compute_array_stats function */ typedef struct { /* Information about array element type */ Oid type_id; /* element type's OID */ Oid eq_opr; /* default equality operator's OID */ Oid coll_id; /* collation to use */ bool typbyval; /* physical properties of element type */ int16 typlen; char typalign; /* * Lookup data for element type's comparison and hash functions (these are * in the type's typcache entry, which we expect to remain valid over the * lifespan of the ANALYZE run) */ FmgrInfo *cmp; FmgrInfo *hash; /* Saved state from std_typanalyze() */ AnalyzeAttrComputeStatsFunc std_compute_stats; void *std_extra_data; } ArrayAnalyzeExtraData; /* * While compute_array_stats is running, we keep a pointer to the extra data * here for use by assorted subroutines. compute_array_stats doesn't * currently need to be re-entrant, so avoiding this is not worth the extra * notational cruft that would be needed. */ static ArrayAnalyzeExtraData *array_extra_data; /* A hash table entry for the Lossy Counting algorithm */ typedef struct { Datum key; /* This is 'e' from the LC algorithm. */ int frequency; /* This is 'f'. */ int delta; /* And this is 'delta'. */ int last_container; /* For de-duplication of array elements. */ } TrackItem; /* A hash table entry for distinct-elements counts */ typedef struct { int count; /* Count of distinct elements in an array */ int frequency; /* Number of arrays seen with this count */ } DECountItem; static void compute_array_stats(VacAttrStats *stats, AnalyzeAttrFetchFunc fetchfunc, int samplerows, double totalrows); static void prune_element_hashtable(HTAB *elements_tab, int b_current); static uint32 element_hash(const void *key, Size keysize); static int element_match(const void *key1, const void *key2, Size keysize); static int element_compare(const void *key1, const void *key2); static int trackitem_compare_frequencies_desc(const void *e1, const void *e2); static int trackitem_compare_element(const void *e1, const void *e2); static int countitem_compare_count(const void *e1, const void *e2); /* * array_typanalyze -- typanalyze function for array columns */ Datum array_typanalyze(PG_FUNCTION_ARGS) { VacAttrStats *stats = (VacAttrStats *) PG_GETARG_POINTER(0); Oid element_typeid; TypeCacheEntry *typentry; ArrayAnalyzeExtraData *extra_data; /* * Call the standard typanalyze function. It may fail to find needed * operators, in which case we also can't do anything, so just fail. */ if (!std_typanalyze(stats)) PG_RETURN_BOOL(false); /* * Check attribute data type is a varlena array (or a domain over one). */ element_typeid = get_base_element_type(stats->attrtypid); if (!OidIsValid(element_typeid)) elog(ERROR, "array_typanalyze was invoked for non-array type %u", stats->attrtypid); /* * Gather information about the element type. If we fail to find * something, return leaving the state from std_typanalyze() in place. */ typentry = lookup_type_cache(element_typeid, TYPECACHE_EQ_OPR | TYPECACHE_CMP_PROC_FINFO | TYPECACHE_HASH_PROC_FINFO); if (!OidIsValid(typentry->eq_opr) || !OidIsValid(typentry->cmp_proc_finfo.fn_oid) || !OidIsValid(typentry->hash_proc_finfo.fn_oid)) PG_RETURN_BOOL(true); /* Store our findings for use by compute_array_stats() */ extra_data = (ArrayAnalyzeExtraData *) palloc(sizeof(ArrayAnalyzeExtraData)); extra_data->type_id = typentry->type_id; extra_data->eq_opr = typentry->eq_opr; extra_data->coll_id = stats->attrcollid; /* collation we should use */ extra_data->typbyval = typentry->typbyval; extra_data->typlen = typentry->typlen; extra_data->typalign = typentry->typalign; extra_data->cmp = &typentry->cmp_proc_finfo; extra_data->hash = &typentry->hash_proc_finfo; /* Save old compute_stats and extra_data for scalar statistics ... */ extra_data->std_compute_stats = stats->compute_stats; extra_data->std_extra_data = stats->extra_data; /* ... and replace with our info */ stats->compute_stats = compute_array_stats; stats->extra_data = extra_data; /* * Note we leave stats->minrows set as std_typanalyze set it. Should it * be increased for array analysis purposes? */ PG_RETURN_BOOL(true); } /* * compute_array_stats() -- compute statistics for an array column * * This function computes statistics useful for determining selectivity of * the array operators <@, &&, and @>. It is invoked by ANALYZE via the * compute_stats hook after sample rows have been collected. * * We also invoke the standard compute_stats function, which will compute * "scalar" statistics relevant to the btree-style array comparison operators. * However, exact duplicates of an entire array may be rare despite many * arrays sharing individual elements. This especially afflicts long arrays, * which are also liable to lack all scalar statistics due to the low * WIDTH_THRESHOLD used in analyze.c. So, in addition to the standard stats, * we find the most common array elements and compute a histogram of distinct * element counts. * * The algorithm used is Lossy Counting, as proposed in the paper "Approximate * frequency counts over data streams" by G. S. Manku and R. Motwani, in * Proceedings of the 28th International Conference on Very Large Data Bases, * Hong Kong, China, August 2002, section 4.2. The paper is available at * http://www.vldb.org/conf/2002/S10P03.pdf * * The Lossy Counting (aka LC) algorithm goes like this: * Let s be the threshold frequency for an item (the minimum frequency we * are interested in) and epsilon the error margin for the frequency. Let D * be a set of triples (e, f, delta), where e is an element value, f is that * element's frequency (actually, its current occurrence count) and delta is * the maximum error in f. We start with D empty and process the elements in * batches of size w. (The batch size is also known as "bucket size" and is * equal to 1/epsilon.) Let the current batch number be b_current, starting * with 1. For each element e we either increment its f count, if it's * already in D, or insert a new triple into D with values (e, 1, b_current * - 1). After processing each batch we prune D, by removing from it all * elements with f + delta <= b_current. After the algorithm finishes we * suppress all elements from D that do not satisfy f >= (s - epsilon) * N, * where N is the total number of elements in the input. We emit the * remaining elements with estimated frequency f/N. The LC paper proves * that this algorithm finds all elements with true frequency at least s, * and that no frequency is overestimated or is underestimated by more than * epsilon. Furthermore, given reasonable assumptions about the input * distribution, the required table size is no more than about 7 times w. * * In the absence of a principled basis for other particular values, we * follow ts_typanalyze() and use parameters s = 0.07/K, epsilon = s/10. * But we leave out the correction for stopwords, which do not apply to * arrays. These parameters give bucket width w = K/0.007 and maximum * expected hashtable size of about 1000 * K. * * Elements may repeat within an array. Since duplicates do not change the * behavior of <@, && or @>, we want to count each element only once per * array. Therefore, we store in the finished pg_statistic entry each * element's frequency as the fraction of all non-null rows that contain it. * We divide the raw counts by nonnull_cnt to get those figures. */ static void compute_array_stats(VacAttrStats *stats, AnalyzeAttrFetchFunc fetchfunc, int samplerows, double totalrows) { ArrayAnalyzeExtraData *extra_data; int num_mcelem; int null_cnt = 0; int null_elem_cnt = 0; int analyzed_rows = 0; /* This is D from the LC algorithm. */ HTAB *elements_tab; HASHCTL elem_hash_ctl; HASH_SEQ_STATUS scan_status; /* This is the current bucket number from the LC algorithm */ int b_current; /* This is 'w' from the LC algorithm */ int bucket_width; int array_no; int64 element_no; TrackItem *item; int slot_idx; HTAB *count_tab; HASHCTL count_hash_ctl; DECountItem *count_item; extra_data = (ArrayAnalyzeExtraData *) stats->extra_data; /* * Invoke analyze.c's standard analysis function to create scalar-style * stats for the column. It will expect its own extra_data pointer, so * temporarily install that. */ stats->extra_data = extra_data->std_extra_data; extra_data->std_compute_stats(stats, fetchfunc, samplerows, totalrows); stats->extra_data = extra_data; /* * Set up static pointer for use by subroutines. We wait till here in * case std_compute_stats somehow recursively invokes us (probably not * possible, but ...) */ array_extra_data = extra_data; /* * We want statistics_target * 10 elements in the MCELEM array. This * multiplier is pretty arbitrary, but is meant to reflect the fact that * the number of individual elements tracked in pg_statistic ought to be * more than the number of values for a simple scalar column. */ num_mcelem = stats->attr->attstattarget * 10; /* * We set bucket width equal to num_mcelem / 0.007 as per the comment * above. */ bucket_width = num_mcelem * 1000 / 7; /* * Create the hashtable. It will be in local memory, so we don't need to * worry about overflowing the initial size. Also we don't need to pay any * attention to locking and memory management. */ MemSet(&elem_hash_ctl, 0, sizeof(elem_hash_ctl)); elem_hash_ctl.keysize = sizeof(Datum); elem_hash_ctl.entrysize = sizeof(TrackItem); elem_hash_ctl.hash = element_hash; elem_hash_ctl.match = element_match; elem_hash_ctl.hcxt = CurrentMemoryContext; elements_tab = hash_create("Analyzed elements table", num_mcelem, &elem_hash_ctl, HASH_ELEM | HASH_FUNCTION | HASH_COMPARE | HASH_CONTEXT); /* hashtable for array distinct elements counts */ MemSet(&count_hash_ctl, 0, sizeof(count_hash_ctl)); count_hash_ctl.keysize = sizeof(int); count_hash_ctl.entrysize = sizeof(DECountItem); count_hash_ctl.hcxt = CurrentMemoryContext; count_tab = hash_create("Array distinct element count table", 64, &count_hash_ctl, HASH_ELEM | HASH_BLOBS | HASH_CONTEXT); /* Initialize counters. */ b_current = 1; element_no = 0; /* Loop over the arrays. */ for (array_no = 0; array_no < samplerows; array_no++) { Datum value; bool isnull; ArrayType *array; int num_elems; Datum *elem_values; bool *elem_nulls; bool null_present; int j; int64 prev_element_no = element_no; int distinct_count; bool count_item_found; vacuum_delay_point(); value = fetchfunc(stats, array_no, &isnull); if (isnull) { /* array is null, just count that */ null_cnt++; continue; } /* Skip too-large values. */ if (toast_raw_datum_size(value) > ARRAY_WIDTH_THRESHOLD) continue; else analyzed_rows++; /* * Now detoast the array if needed, and deconstruct into datums. */ array = DatumGetArrayTypeP(value); Assert(ARR_ELEMTYPE(array) == extra_data->type_id); deconstruct_array(array, extra_data->type_id, extra_data->typlen, extra_data->typbyval, extra_data->typalign, &elem_values, &elem_nulls, &num_elems); /* * We loop through the elements in the array and add them to our * tracking hashtable. */ null_present = false; for (j = 0; j < num_elems; j++) { Datum elem_value; bool found; /* No null element processing other than flag setting here */ if (elem_nulls[j]) { null_present = true; continue; } /* Lookup current element in hashtable, adding it if new */ elem_value = elem_values[j]; item = (TrackItem *) hash_search(elements_tab, (const void *) &elem_value, HASH_ENTER, &found); if (found) { /* The element value is already on the tracking list */ /* * The operators we assist ignore duplicate array elements, so * count a given distinct element only once per array. */ if (item->last_container == array_no) continue; item->frequency++; item->last_container = array_no; } else { /* Initialize new tracking list element */ /* * If element type is pass-by-reference, we must copy it into * palloc'd space, so that we can release the array below. (We * do this so that the space needed for element values is * limited by the size of the hashtable; if we kept all the * array values around, it could be much more.) */ item->key = datumCopy(elem_value, extra_data->typbyval, extra_data->typlen); item->frequency = 1; item->delta = b_current - 1; item->last_container = array_no; } /* element_no is the number of elements processed (ie N) */ element_no++; /* We prune the D structure after processing each bucket */ if (element_no % bucket_width == 0) { prune_element_hashtable(elements_tab, b_current); b_current++; } } /* Count null element presence once per array. */ if (null_present) null_elem_cnt++; /* Update frequency of the particular array distinct element count. */ distinct_count = (int) (element_no - prev_element_no); count_item = (DECountItem *) hash_search(count_tab, &distinct_count, HASH_ENTER, &count_item_found); if (count_item_found) count_item->frequency++; else count_item->frequency = 1; /* Free memory allocated while detoasting. */ if (PointerGetDatum(array) != value) pfree(array); pfree(elem_values); pfree(elem_nulls); } /* Skip pg_statistic slots occupied by standard statistics */ slot_idx = 0; while (slot_idx < STATISTIC_NUM_SLOTS && stats->stakind[slot_idx] != 0) slot_idx++; if (slot_idx > STATISTIC_NUM_SLOTS - 2) elog(ERROR, "insufficient pg_statistic slots for array stats"); /* We can only compute real stats if we found some non-null values. */ if (analyzed_rows > 0) { int nonnull_cnt = analyzed_rows; int count_items_count; int i; TrackItem **sort_table; int track_len; int64 cutoff_freq; int64 minfreq, maxfreq; /* * We assume the standard stats code already took care of setting * stats_valid, stanullfrac, stawidth, stadistinct. We'd have to * re-compute those values if we wanted to not store the standard * stats. */ /* * Construct an array of the interesting hashtable items, that is, * those meeting the cutoff frequency (s - epsilon)*N. Also identify * the minimum and maximum frequencies among these items. * * Since epsilon = s/10 and bucket_width = 1/epsilon, the cutoff * frequency is 9*N / bucket_width. */ cutoff_freq = 9 * element_no / bucket_width; i = hash_get_num_entries(elements_tab); /* surely enough space */ sort_table = (TrackItem **) palloc(sizeof(TrackItem *) * i); hash_seq_init(&scan_status, elements_tab); track_len = 0; minfreq = element_no; maxfreq = 0; while ((item = (TrackItem *) hash_seq_search(&scan_status)) != NULL) { if (item->frequency > cutoff_freq) { sort_table[track_len++] = item; minfreq = Min(minfreq, item->frequency); maxfreq = Max(maxfreq, item->frequency); } } Assert(track_len <= i); /* emit some statistics for debug purposes */ elog(DEBUG3, "compute_array_stats: target # mces = %d, " "bucket width = %d, " "# elements = " INT64_FORMAT ", hashtable size = %d, " "usable entries = %d", num_mcelem, bucket_width, element_no, i, track_len); /* * If we obtained more elements than we really want, get rid of those * with least frequencies. The easiest way is to qsort the array into * descending frequency order and truncate the array. */ if (num_mcelem < track_len) { qsort(sort_table, track_len, sizeof(TrackItem *), trackitem_compare_frequencies_desc); /* reset minfreq to the smallest frequency we're keeping */ minfreq = sort_table[num_mcelem - 1]->frequency; } else num_mcelem = track_len; /* Generate MCELEM slot entry */ if (num_mcelem > 0) { MemoryContext old_context; Datum *mcelem_values; float4 *mcelem_freqs; /* * We want to store statistics sorted on the element value using * the element type's default comparison function. This permits * fast binary searches in selectivity estimation functions. */ qsort(sort_table, num_mcelem, sizeof(TrackItem *), trackitem_compare_element); /* Must copy the target values into anl_context */ old_context = MemoryContextSwitchTo(stats->anl_context); /* * We sorted statistics on the element value, but we want to be * able to find the minimal and maximal frequencies without going * through all the values. We also want the frequency of null * elements. Store these three values at the end of mcelem_freqs. */ mcelem_values = (Datum *) palloc(num_mcelem * sizeof(Datum)); mcelem_freqs = (float4 *) palloc((num_mcelem + 3) * sizeof(float4)); /* * See comments above about use of nonnull_cnt as the divisor for * the final frequency estimates. */ for (i = 0; i < num_mcelem; i++) { TrackItem *item = sort_table[i]; mcelem_values[i] = datumCopy(item->key, extra_data->typbyval, extra_data->typlen); mcelem_freqs[i] = (double) item->frequency / (double) nonnull_cnt; } mcelem_freqs[i++] = (double) minfreq / (double) nonnull_cnt; mcelem_freqs[i++] = (double) maxfreq / (double) nonnull_cnt; mcelem_freqs[i++] = (double) null_elem_cnt / (double) nonnull_cnt; MemoryContextSwitchTo(old_context); stats->stakind[slot_idx] = STATISTIC_KIND_MCELEM; stats->staop[slot_idx] = extra_data->eq_opr; stats->stacoll[slot_idx] = extra_data->coll_id; stats->stanumbers[slot_idx] = mcelem_freqs; /* See above comment about extra stanumber entries */ stats->numnumbers[slot_idx] = num_mcelem + 3; stats->stavalues[slot_idx] = mcelem_values; stats->numvalues[slot_idx] = num_mcelem; /* We are storing values of element type */ stats->statypid[slot_idx] = extra_data->type_id; stats->statyplen[slot_idx] = extra_data->typlen; stats->statypbyval[slot_idx] = extra_data->typbyval; stats->statypalign[slot_idx] = extra_data->typalign; slot_idx++; } /* Generate DECHIST slot entry */ count_items_count = hash_get_num_entries(count_tab); if (count_items_count > 0) { int num_hist = stats->attr->attstattarget; DECountItem **sorted_count_items; int j; int delta; int64 frac; float4 *hist; /* num_hist must be at least 2 for the loop below to work */ num_hist = Max(num_hist, 2); /* * Create an array of DECountItem pointers, and sort them into * increasing count order. */ sorted_count_items = (DECountItem **) palloc(sizeof(DECountItem *) * count_items_count); hash_seq_init(&scan_status, count_tab); j = 0; while ((count_item = (DECountItem *) hash_seq_search(&scan_status)) != NULL) { sorted_count_items[j++] = count_item; } qsort(sorted_count_items, count_items_count, sizeof(DECountItem *), countitem_compare_count); /* * Prepare to fill stanumbers with the histogram, followed by the * average count. This array must be stored in anl_context. */ hist = (float4 *) MemoryContextAlloc(stats->anl_context, sizeof(float4) * (num_hist + 1)); hist[num_hist] = (double) element_no / (double) nonnull_cnt; /*---------- * Construct the histogram of distinct-element counts (DECs). * * The object of this loop is to copy the min and max DECs to * hist[0] and hist[num_hist - 1], along with evenly-spaced DECs * in between (where "evenly-spaced" is with reference to the * whole input population of arrays). If we had a complete sorted * array of DECs, one per analyzed row, the i'th hist value would * come from DECs[i * (analyzed_rows - 1) / (num_hist - 1)] * (compare the histogram-making loop in compute_scalar_stats()). * But instead of that we have the sorted_count_items[] array, * which holds unique DEC values with their frequencies (that is, * a run-length-compressed version of the full array). So we * control advancing through sorted_count_items[] with the * variable "frac", which is defined as (x - y) * (num_hist - 1), * where x is the index in the notional DECs array corresponding * to the start of the next sorted_count_items[] element's run, * and y is the index in DECs from which we should take the next * histogram value. We have to advance whenever x <= y, that is * frac <= 0. The x component is the sum of the frequencies seen * so far (up through the current sorted_count_items[] element), * and of course y * (num_hist - 1) = i * (analyzed_rows - 1), * per the subscript calculation above. (The subscript calculation * implies dropping any fractional part of y; in this formulation * that's handled by not advancing until frac reaches 1.) * * Even though frac has a bounded range, it could overflow int32 * when working with very large statistics targets, so we do that * math in int64. *---------- */ delta = analyzed_rows - 1; j = 0; /* current index in sorted_count_items */ /* Initialize frac for sorted_count_items[0]; y is initially 0 */ frac = (int64) sorted_count_items[0]->frequency * (num_hist - 1); for (i = 0; i < num_hist; i++) { while (frac <= 0) { /* Advance, and update x component of frac */ j++; frac += (int64) sorted_count_items[j]->frequency * (num_hist - 1); } hist[i] = sorted_count_items[j]->count; frac -= delta; /* update y for upcoming i increment */ } Assert(j == count_items_count - 1); stats->stakind[slot_idx] = STATISTIC_KIND_DECHIST; stats->staop[slot_idx] = extra_data->eq_opr; stats->stacoll[slot_idx] = extra_data->coll_id; stats->stanumbers[slot_idx] = hist; stats->numnumbers[slot_idx] = num_hist + 1; slot_idx++; } } /* * We don't need to bother cleaning up any of our temporary palloc's. The * hashtable should also go away, as it used a child memory context. */ } /* * A function to prune the D structure from the Lossy Counting algorithm. * Consult compute_tsvector_stats() for wider explanation. */ static void prune_element_hashtable(HTAB *elements_tab, int b_current) { HASH_SEQ_STATUS scan_status; TrackItem *item; hash_seq_init(&scan_status, elements_tab); while ((item = (TrackItem *) hash_seq_search(&scan_status)) != NULL) { if (item->frequency + item->delta <= b_current) { Datum value = item->key; if (hash_search(elements_tab, (const void *) &item->key, HASH_REMOVE, NULL) == NULL) elog(ERROR, "hash table corrupted"); /* We should free memory if element is not passed by value */ if (!array_extra_data->typbyval) pfree(DatumGetPointer(value)); } } } /* * Hash function for elements. * * We use the element type's default hash opclass, and the column collation * if the type is collation-sensitive. */ static uint32 element_hash(const void *key, Size keysize) { Datum d = *((const Datum *) key); Datum h; h = FunctionCall1Coll(array_extra_data->hash, array_extra_data->coll_id, d); return DatumGetUInt32(h); } /* * Matching function for elements, to be used in hashtable lookups. */ static int element_match(const void *key1, const void *key2, Size keysize) { /* The keysize parameter is superfluous here */ return element_compare(key1, key2); } /* * Comparison function for elements. * * We use the element type's default btree opclass, and the column collation * if the type is collation-sensitive. * * XXX consider using SortSupport infrastructure */ static int element_compare(const void *key1, const void *key2) { Datum d1 = *((const Datum *) key1); Datum d2 = *((const Datum *) key2); Datum c; c = FunctionCall2Coll(array_extra_data->cmp, array_extra_data->coll_id, d1, d2); return DatumGetInt32(c); } /* * qsort() comparator for sorting TrackItems by 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 by element values */ static int trackitem_compare_element(const void *e1, const void *e2) { const TrackItem *const *t1 = (const TrackItem *const *) e1; const TrackItem *const *t2 = (const TrackItem *const *) e2; return element_compare(&(*t1)->key, &(*t2)->key); } /* * qsort() comparator for sorting DECountItems by count */ static int countitem_compare_count(const void *e1, const void *e2) { const DECountItem *const *t1 = (const DECountItem *const *) e1; const DECountItem *const *t2 = (const DECountItem *const *) e2; if ((*t1)->count < (*t2)->count) return -1; else if ((*t1)->count == (*t2)->count) return 0; else return 1; }