path: root/contrib/pg_stat_statements/pg_stat_statements.c

/*-------------------------------------------------------------------------
 *
 * pg_stat_statements.c
 *		Track statement execution times across a whole database cluster.
 *
 * Note about locking issues: to create or delete an entry in the shared
 * hashtable, one must hold pgss->lock exclusively.  Modifying any field
 * in an entry except the counters requires the same.  To look up an entry,
 * one must hold the lock shared.  To read or update the counters within
 * an entry, one must hold the lock shared or exclusive (so the entry doesn't
 * disappear!) and also take the entry's mutex spinlock.
 *
 *
 * Copyright (c) 2008-2009, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *	  $PostgreSQL: pgsql/contrib/pg_stat_statements/pg_stat_statements.c,v 1.3.2.3 2009/12/29 17:41:09 heikki Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <unistd.h>

#include "access/hash.h"
#include "catalog/pg_type.h"
#include "executor/executor.h"
#include "executor/instrument.h"
#include "mb/pg_wchar.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/spin.h"
#include "utils/builtins.h"
#include "utils/hsearch.h"
#include "utils/guc.h"


PG_MODULE_MAGIC;

/* Location of stats file */
#define PGSS_DUMP_FILE	"global/pg_stat_statements.stat"

/* This constant defines the magic number in the stats file header */
static const uint32 PGSS_FILE_HEADER = 0x20081202;

/* XXX: Should USAGE_EXEC reflect execution time and/or buffer usage? */
#define USAGE_EXEC(duration)	(1.0)
#define USAGE_INIT				(1.0)	/* including initial planning */
#define USAGE_DECREASE_FACTOR	(0.99)	/* decreased every entry_dealloc */
#define USAGE_DEALLOC_PERCENT	5		/* free this % of entries at once */

/*
 * Hashtable key that defines the identity of a hashtable entry.  The
 * hash comparators do not assume that the query string is null-terminated;
 * this lets us search for an mbcliplen'd string without copying it first.
 *
 * Presently, the query encoding is fully determined by the source database
 * and so we don't really need it to be in the key.  But that might not always
 * be true. Anyway it's notationally convenient to pass it as part of the key.
 */
typedef struct pgssHashKey
{
	Oid			userid;			/* user OID */
	Oid			dbid;			/* database OID */
	int			encoding;		/* query encoding */
	int			query_len;		/* # of valid bytes in query string */
	const char *query_ptr;		/* query string proper */
} pgssHashKey;

/*
 * The actual stats counters kept within pgssEntry.
 */
typedef struct Counters
{
	int64		calls;			/* # of times executed */
	double		total_time;		/* total execution time in seconds */
	int64		rows;			/* total # of retrieved or affected rows */
	double		usage;			/* usage factor */
} Counters;

/*
 * Statistics per statement
 *
 * NB: see the file read/write code before changing field order here.
 */
typedef struct pgssEntry
{
	pgssHashKey key;			/* hash key of entry - MUST BE FIRST */
	Counters	counters;		/* the statistics for this query */
	slock_t		mutex;			/* protects the counters only */
	char		query[1];		/* VARIABLE LENGTH ARRAY - MUST BE LAST */
	/* Note: the allocated length of query[] is actually pgss->query_size */
} pgssEntry;

/*
 * Global shared state
 */
typedef struct pgssSharedState
{
	LWLockId	lock;			/* protects hashtable search/modification */
	int			query_size;		/* max query length in bytes */
} pgssSharedState;

/*---- Local variables ----*/

/* Current nesting depth of ExecutorRun calls */
static int	nested_level = 0;

/* Saved hook values in case of unload */
static shmem_startup_hook_type prev_shmem_startup_hook = NULL;
static ExecutorStart_hook_type prev_ExecutorStart = NULL;
static ExecutorRun_hook_type prev_ExecutorRun = NULL;
static ExecutorEnd_hook_type prev_ExecutorEnd = NULL;

/* Links to shared memory state */
static pgssSharedState *pgss = NULL;
static HTAB *pgss_hash = NULL;

/*---- GUC variables ----*/

typedef enum
{
	PGSS_TRACK_NONE,			/* track no statements */
	PGSS_TRACK_TOP,				/* only top level statements */
	PGSS_TRACK_ALL,				/* all statements, including nested ones */
} PGSSTrackLevel;

static const struct config_enum_entry track_options[] = {
	{"none", PGSS_TRACK_NONE, false},
	{"top", PGSS_TRACK_TOP, false},
	{"all", PGSS_TRACK_ALL, false},
	{NULL, 0, false}
};

static int	pgss_max;			/* max # statements to track */
static int	pgss_track;			/* tracking level */
static bool pgss_save;			/* whether to save stats across shutdown */


#define pgss_enabled() \
	(pgss_track == PGSS_TRACK_ALL || \
	(pgss_track == PGSS_TRACK_TOP && nested_level == 0))

/*---- Function declarations ----*/

void		_PG_init(void);
void		_PG_fini(void);

Datum		pg_stat_statements_reset(PG_FUNCTION_ARGS);
Datum		pg_stat_statements(PG_FUNCTION_ARGS);

PG_FUNCTION_INFO_V1(pg_stat_statements_reset);
PG_FUNCTION_INFO_V1(pg_stat_statements);

static void pgss_shmem_startup(void);
static void pgss_shmem_shutdown(int code, Datum arg);
static void pgss_ExecutorStart(QueryDesc *queryDesc, int eflags);
static void pgss_ExecutorRun(QueryDesc *queryDesc,
				 ScanDirection direction,
				 long count);
static void pgss_ExecutorEnd(QueryDesc *queryDesc);
static uint32 pgss_hash_fn(const void *key, Size keysize);
static int	pgss_match_fn(const void *key1, const void *key2, Size keysize);
static void pgss_store(const char *query,
		   const Instrumentation *instr, uint32 rows);
static Size pgss_memsize(void);
static pgssEntry *entry_alloc(pgssHashKey *key);
static void entry_dealloc(void);
static void entry_reset(void);


/*
 * Module load callback
 */
void
_PG_init(void)
{
	/*
	 * In order to create our shared memory area, we have to be loaded via
	 * shared_preload_libraries.  If not, fall out without hooking into any of
	 * the main system.  (We don't throw error here because it seems useful to
	 * allow the pg_stat_statements functions to be created even when the
	 * module isn't active.  The functions must protect themselves against
	 * being called then, however.)
	 */
	if (!process_shared_preload_libraries_in_progress)
		return;

	/*
	 * Define (or redefine) custom GUC variables.
	 */
	DefineCustomIntVariable("pg_stat_statements.max",
	  "Sets the maximum number of statements tracked by pg_stat_statements.",
							NULL,
							&pgss_max,
							1000,
							100,
							INT_MAX,
							PGC_POSTMASTER,
							0,
							NULL,
							NULL);

	DefineCustomEnumVariable("pg_stat_statements.track",
			   "Selects which statements are tracked by pg_stat_statements.",
							 NULL,
							 &pgss_track,
							 PGSS_TRACK_TOP,
							 track_options,
							 PGC_SUSET,
							 0,
							 NULL,
							 NULL);

	DefineCustomBoolVariable("pg_stat_statements.save",
			   "Save pg_stat_statements statistics across server shutdowns.",
							 NULL,
							 &pgss_save,
							 true,
							 PGC_SIGHUP,
							 0,
							 NULL,
							 NULL);

	EmitWarningsOnPlaceholders("pg_stat_statements");

	/*
	 * Request additional shared resources.  (These are no-ops if we're not in
	 * the postmaster process.)  We'll allocate or attach to the shared
	 * resources in pgss_shmem_startup().
	 */
	RequestAddinShmemSpace(pgss_memsize());
	RequestAddinLWLocks(1);

	/*
	 * Install hooks.
	 */
	prev_shmem_startup_hook = shmem_startup_hook;
	shmem_startup_hook = pgss_shmem_startup;
	prev_ExecutorStart = ExecutorStart_hook;
	ExecutorStart_hook = pgss_ExecutorStart;
	prev_ExecutorRun = ExecutorRun_hook;
	ExecutorRun_hook = pgss_ExecutorRun;
	prev_ExecutorEnd = ExecutorEnd_hook;
	ExecutorEnd_hook = pgss_ExecutorEnd;
}

/*
 * Module unload callback
 */
void
_PG_fini(void)
{
	/* Uninstall hooks. */
	ExecutorStart_hook = prev_ExecutorStart;
	ExecutorRun_hook = prev_ExecutorRun;
	ExecutorEnd_hook = prev_ExecutorEnd;
	shmem_startup_hook = prev_shmem_startup_hook;
}

/*
 * shmem_startup hook: allocate or attach to shared memory,
 * then load any pre-existing statistics from file.
 */
static void
pgss_shmem_startup(void)
{
	bool		found;
	HASHCTL		info;
	FILE	   *file;
	uint32		header;
	int32		num;
	int32		i;
	int			query_size;
	int			buffer_size;
	char	   *buffer = NULL;

	if (prev_shmem_startup_hook)
		prev_shmem_startup_hook();

	/* reset in case this is a restart within the postmaster */
	pgss = NULL;
	pgss_hash = NULL;

	/*
	 * Create or attach to the shared memory state, including hash table
	 */
	LWLockAcquire(AddinShmemInitLock, LW_EXCLUSIVE);

	pgss = ShmemInitStruct("pg_stat_statements",
						   sizeof(pgssSharedState),
						   &found);
	if (!pgss)
		elog(ERROR, "out of shared memory");

	if (!found)
	{
		/* First time through ... */
		pgss->lock = LWLockAssign();
		pgss->query_size = pgstat_track_activity_query_size;
	}

	/* Be sure everyone agrees on the hash table entry size */
	query_size = pgss->query_size;

	memset(&info, 0, sizeof(info));
	info.keysize = sizeof(pgssHashKey);
	info.entrysize = offsetof(pgssEntry, query) +query_size;
	info.hash = pgss_hash_fn;
	info.match = pgss_match_fn;
	pgss_hash = ShmemInitHash("pg_stat_statements hash",
							  pgss_max, pgss_max,
							  &info,
							  HASH_ELEM | HASH_FUNCTION | HASH_COMPARE);
	if (!pgss_hash)
		elog(ERROR, "out of shared memory");

	LWLockRelease(AddinShmemInitLock);

	/*
	 * If we're in the postmaster (or a standalone backend...), set up a shmem
	 * exit hook to dump the statistics to disk.
	 */
	if (!IsUnderPostmaster)
		on_shmem_exit(pgss_shmem_shutdown, (Datum) 0);

	/*
	 * Attempt to load old statistics from the dump file, if this is the
	 * first time through and we weren't told not to.
	 */
	if (found || !pgss_save)
		return;

	/*
	 * Note: we don't bother with locks here, because there should be no other
	 * processes running when this code is reached.
	 */
	file = AllocateFile(PGSS_DUMP_FILE, PG_BINARY_R);
	if (file == NULL)
	{
		if (errno == ENOENT)
			return;				/* ignore not-found error */
		goto error;
	}

	buffer_size = query_size;
	buffer = (char *) palloc(buffer_size);

	if (fread(&header, sizeof(uint32), 1, file) != 1 ||
		header != PGSS_FILE_HEADER ||
		fread(&num, sizeof(int32), 1, file) != 1)
		goto error;

	for (i = 0; i < num; i++)
	{
		pgssEntry	temp;
		pgssEntry  *entry;

		if (fread(&temp, offsetof(pgssEntry, mutex), 1, file) != 1)
			goto error;

		/* Encoding is the only field we can easily sanity-check */
		if (!PG_VALID_BE_ENCODING(temp.key.encoding))
			goto error;

		/* Previous incarnation might have had a larger query_size */
		if (temp.key.query_len >= buffer_size)
		{
			buffer = (char *) repalloc(buffer, temp.key.query_len + 1);
			buffer_size = temp.key.query_len + 1;
		}

		if (fread(buffer, 1, temp.key.query_len, file) != temp.key.query_len)
			goto error;
		buffer[temp.key.query_len] = '\0';

		/* Clip to available length if needed */
		if (temp.key.query_len >= query_size)
			temp.key.query_len = pg_encoding_mbcliplen(temp.key.encoding,
													   buffer,
													   temp.key.query_len,
													   query_size - 1);
		temp.key.query_ptr = buffer;

		/* make the hashtable entry (discards old entries if too many) */
		entry = entry_alloc(&temp.key);

		/* copy in the actual stats */
		entry->counters = temp.counters;
	}

	pfree(buffer);
	FreeFile(file);
	return;

error:
	ereport(LOG,
			(errcode_for_file_access(),
			 errmsg("could not read pg_stat_statement file \"%s\": %m",
					PGSS_DUMP_FILE)));
	if (buffer)
		pfree(buffer);
	if (file)
		FreeFile(file);
	/* If possible, throw away the bogus file; ignore any error */
	unlink(PGSS_DUMP_FILE);
}

/*
 * shmem_shutdown hook: Dump statistics into file.
 *
 * Note: we don't bother with acquiring lock, because there should be no
 * other processes running when this is called.
 */
static void
pgss_shmem_shutdown(int code, Datum arg)
{
	FILE	   *file;
	HASH_SEQ_STATUS hash_seq;
	int32		num_entries;
	pgssEntry  *entry;

	/* Don't try to dump during a crash. */
	if (code)
		return;

	/* Safety check ... shouldn't get here unless shmem is set up. */
	if (!pgss || !pgss_hash)
		return;

	/* Don't dump if told not to. */
	if (!pgss_save)
		return;

	file = AllocateFile(PGSS_DUMP_FILE, PG_BINARY_W);
	if (file == NULL)
		goto error;

	if (fwrite(&PGSS_FILE_HEADER, sizeof(uint32), 1, file) != 1)
		goto error;
	num_entries = hash_get_num_entries(pgss_hash);
	if (fwrite(&num_entries, sizeof(int32), 1, file) != 1)
		goto error;

	hash_seq_init(&hash_seq, pgss_hash);
	while ((entry = hash_seq_search(&hash_seq)) != NULL)
	{
		int			len = entry->key.query_len;

		if (fwrite(entry, offsetof(pgssEntry, mutex), 1, file) != 1 ||
			fwrite(entry->query, 1, len, file) != len)
			goto error;
	}

	if (FreeFile(file))
	{
		file = NULL;
		goto error;
	}

	return;

error:
	ereport(LOG,
			(errcode_for_file_access(),
			 errmsg("could not write pg_stat_statement file \"%s\": %m",
					PGSS_DUMP_FILE)));
	if (file)
		FreeFile(file);
	unlink(PGSS_DUMP_FILE);
}

/*
 * ExecutorStart hook: start up tracking if needed
 */
static void
pgss_ExecutorStart(QueryDesc *queryDesc, int eflags)
{
	if (prev_ExecutorStart)
		prev_ExecutorStart(queryDesc, eflags);
	else
		standard_ExecutorStart(queryDesc, eflags);

	if (pgss_enabled())
	{
		/*
		 * Set up to track total elapsed time in ExecutorRun.  Make sure the
		 * space is allocated in the per-query context so it will go away at
		 * ExecutorEnd.
		 */
		if (queryDesc->totaltime == NULL)
		{
			MemoryContext oldcxt;

			oldcxt = MemoryContextSwitchTo(queryDesc->estate->es_query_cxt);
			queryDesc->totaltime = InstrAlloc(1);
			MemoryContextSwitchTo(oldcxt);
		}
	}
}

/*
 * ExecutorRun hook: all we need do is track nesting depth
 */
static void
pgss_ExecutorRun(QueryDesc *queryDesc, ScanDirection direction, long count)
{
	nested_level++;
	PG_TRY();
	{
		if (prev_ExecutorRun)
			prev_ExecutorRun(queryDesc, direction, count);
		else
			standard_ExecutorRun(queryDesc, direction, count);
		nested_level--;
	}
	PG_CATCH();
	{
		nested_level--;
		PG_RE_THROW();
	}
	PG_END_TRY();
}

/*
 * ExecutorEnd hook: store results if needed
 */
static void
pgss_ExecutorEnd(QueryDesc *queryDesc)
{
	if (queryDesc->totaltime && pgss_enabled())
	{
		/*
		 * Make sure stats accumulation is done.  (Note: it's okay if several
		 * levels of hook all do this.)
		 */
		InstrEndLoop(queryDesc->totaltime);

		pgss_store(queryDesc->sourceText,
				   queryDesc->totaltime,
				   queryDesc->estate->es_processed);
	}

	if (prev_ExecutorEnd)
		prev_ExecutorEnd(queryDesc);
	else
		standard_ExecutorEnd(queryDesc);
}

/*
 * Calculate hash value for a key
 */
static uint32
pgss_hash_fn(const void *key, Size keysize)
{
	const pgssHashKey *k = (const pgssHashKey *) key;

	/* we don't bother to include encoding in the hash */
	return hash_uint32((uint32) k->userid) ^
		hash_uint32((uint32) k->dbid) ^
		DatumGetUInt32(hash_any((const unsigned char *) k->query_ptr,
								k->query_len));
}

/*
 * Compare two keys - zero means match
 */
static int
pgss_match_fn(const void *key1, const void *key2, Size keysize)
{
	const pgssHashKey *k1 = (const pgssHashKey *) key1;
	const pgssHashKey *k2 = (const pgssHashKey *) key2;

	if (k1->userid == k2->userid &&
		k1->dbid == k2->dbid &&
		k1->encoding == k2->encoding &&
		k1->query_len == k2->query_len &&
		memcmp(k1->query_ptr, k2->query_ptr, k1->query_len) == 0)
		return 0;
	else
		return 1;
}

/*
 * Store some statistics for a statement.
 */
static void
pgss_store(const char *query, const Instrumentation *instr, uint32 rows)
{
	pgssHashKey key;
	double		usage;
	pgssEntry  *entry;

	Assert(query != NULL);

	/* Safety check... */
	if (!pgss || !pgss_hash)
		return;

	/* Set up key for hashtable search */
	key.userid = GetUserId();
	key.dbid = MyDatabaseId;
	key.encoding = GetDatabaseEncoding();
	key.query_len = strlen(query);
	if (key.query_len >= pgss->query_size)
		key.query_len = pg_encoding_mbcliplen(key.encoding,
											  query,
											  key.query_len,
											  pgss->query_size - 1);
	key.query_ptr = query;

	usage = USAGE_EXEC(duration);

	/* Lookup the hash table entry with shared lock. */
	LWLockAcquire(pgss->lock, LW_SHARED);

	entry = (pgssEntry *) hash_search(pgss_hash, &key, HASH_FIND, NULL);
	if (!entry)
	{
		/* Must acquire exclusive lock to add a new entry. */
		LWLockRelease(pgss->lock);
		LWLockAcquire(pgss->lock, LW_EXCLUSIVE);
		entry = entry_alloc(&key);
	}

	/* Grab the spinlock while updating the counters. */
	{
		volatile pgssEntry *e = (volatile pgssEntry *) entry;

		SpinLockAcquire(&e->mutex);
		e->counters.calls += 1;
		e->counters.total_time += instr->total;
		e->counters.rows += rows;
		e->counters.usage += usage;
		SpinLockRelease(&e->mutex);
	}

	LWLockRelease(pgss->lock);
}

/*
 * Reset all statement statistics.
 */
Datum
pg_stat_statements_reset(PG_FUNCTION_ARGS)
{
	if (!pgss || !pgss_hash)
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
				 errmsg("pg_stat_statements must be loaded via shared_preload_libraries")));
	entry_reset();
	PG_RETURN_VOID();
}

#define PG_STAT_STATEMENTS_COLS		6

/*
 * Retrieve statement statistics.
 */
Datum
pg_stat_statements(PG_FUNCTION_ARGS)
{
	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
	TupleDesc	tupdesc;
	Tuplestorestate *tupstore;
	MemoryContext per_query_ctx;
	MemoryContext oldcontext;
	Oid			userid = GetUserId();
	bool		is_superuser = superuser();
	HASH_SEQ_STATUS hash_seq;
	pgssEntry  *entry;

	if (!pgss || !pgss_hash)
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
				 errmsg("pg_stat_statements must be loaded via shared_preload_libraries")));

	/* check to see if caller supports us returning a tuplestore */
	if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
		ereport(ERROR,
				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
				 errmsg("set-valued function called in context that cannot accept a set")));
	if (!(rsinfo->allowedModes & SFRM_Materialize))
		ereport(ERROR,
				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
				 errmsg("materialize mode required, but it is not " \
						"allowed in this context")));

	per_query_ctx = rsinfo->econtext->ecxt_per_query_memory;
	oldcontext = MemoryContextSwitchTo(per_query_ctx);

	tupdesc = CreateTemplateTupleDesc(PG_STAT_STATEMENTS_COLS, false);
	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "userid",
					   OIDOID, -1, 0);
	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "dbid",
					   OIDOID, -1, 0);
	TupleDescInitEntry(tupdesc, (AttrNumber) 3, "query",
					   TEXTOID, -1, 0);
	TupleDescInitEntry(tupdesc, (AttrNumber) 4, "calls",
					   INT8OID, -1, 0);
	TupleDescInitEntry(tupdesc, (AttrNumber) 5, "total_time",
					   FLOAT8OID, -1, 0);
	TupleDescInitEntry(tupdesc, (AttrNumber) 6, "rows",
					   INT8OID, -1, 0);

	tupstore = tuplestore_begin_heap(true, false, work_mem);
	rsinfo->returnMode = SFRM_Materialize;
	rsinfo->setResult = tupstore;
	rsinfo->setDesc = tupdesc;

	MemoryContextSwitchTo(oldcontext);

	LWLockAcquire(pgss->lock, LW_SHARED);

	hash_seq_init(&hash_seq, pgss_hash);
	while ((entry = hash_seq_search(&hash_seq)) != NULL)
	{
		Datum		values[PG_STAT_STATEMENTS_COLS];
		bool		nulls[PG_STAT_STATEMENTS_COLS];
		int			i = 0;
		Counters	tmp;

		memset(values, 0, sizeof(values));
		memset(nulls, 0, sizeof(nulls));

		values[i++] = ObjectIdGetDatum(entry->key.userid);
		values[i++] = ObjectIdGetDatum(entry->key.dbid);

		if (is_superuser || entry->key.userid == userid)
		{
			char	   *qstr;

			qstr = (char *)
				pg_do_encoding_conversion((unsigned char *) entry->query,
										  entry->key.query_len,
										  entry->key.encoding,
										  GetDatabaseEncoding());
			values[i++] = CStringGetTextDatum(qstr);
			if (qstr != entry->query)
				pfree(qstr);
		}
		else
			values[i++] = CStringGetTextDatum("<insufficient privilege>");

		/* copy counters to a local variable to keep locking time short */
		{
			volatile pgssEntry *e = (volatile pgssEntry *) entry;

			SpinLockAcquire(&e->mutex);
			tmp = e->counters;
			SpinLockRelease(&e->mutex);
		}

		values[i++] = Int64GetDatumFast(tmp.calls);
		values[i++] = Float8GetDatumFast(tmp.total_time);
		values[i++] = Int64GetDatumFast(tmp.rows);

		Assert(i == PG_STAT_STATEMENTS_COLS);

		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
	}

	LWLockRelease(pgss->lock);

	/* clean up and return the tuplestore */
	tuplestore_donestoring(tupstore);

	return (Datum) 0;
}

/*
 * Estimate shared memory space needed.
 */
static Size
pgss_memsize(void)
{
	Size		size;
	Size		entrysize;

	size = MAXALIGN(sizeof(pgssSharedState));
	entrysize = offsetof(pgssEntry, query) +pgstat_track_activity_query_size;
	size = add_size(size, hash_estimate_size(pgss_max, entrysize));

	return size;
}

/*
 * Allocate a new hashtable entry.
 * caller must hold an exclusive lock on pgss->lock
 *
 * Note: despite needing exclusive lock, it's not an error for the target
 * entry to already exist.	This is because pgss_store releases and
 * reacquires lock after failing to find a match; so someone else could
 * have made the entry while we waited to get exclusive lock.
 */
static pgssEntry *
entry_alloc(pgssHashKey *key)
{
	pgssEntry  *entry;
	bool		found;

	/* Caller must have clipped query properly */
	Assert(key->query_len < pgss->query_size);

	/* Make space if needed */
	while (hash_get_num_entries(pgss_hash) >= pgss_max)
		entry_dealloc();

	/* Find or create an entry with desired hash code */
	entry = (pgssEntry *) hash_search(pgss_hash, key, HASH_ENTER, &found);

	if (!found)
	{
		/* New entry, initialize it */

		/* dynahash tried to copy the key for us, but must fix query_ptr */
		entry->key.query_ptr = entry->query;
		/* reset the statistics */
		memset(&entry->counters, 0, sizeof(Counters));
		entry->counters.usage = USAGE_INIT;
		/* re-initialize the mutex each time ... we assume no one using it */
		SpinLockInit(&entry->mutex);
		/* ... and don't forget the query text */
		memcpy(entry->query, key->query_ptr, key->query_len);
		entry->query[key->query_len] = '\0';
	}

	return entry;
}

/*
 * qsort comparator for sorting into increasing usage order
 */
static int
entry_cmp(const void *lhs, const void *rhs)
{
	double		l_usage = (*(const pgssEntry **) lhs)->counters.usage;
	double		r_usage = (*(const pgssEntry **) rhs)->counters.usage;

	if (l_usage < r_usage)
		return -1;
	else if (l_usage > r_usage)
		return +1;
	else
		return 0;
}

/*
 * Deallocate least used entries.
 * Caller must hold an exclusive lock on pgss->lock.
 */
static void
entry_dealloc(void)
{
	HASH_SEQ_STATUS hash_seq;
	pgssEntry **entries;
	pgssEntry  *entry;
	int			nvictims;
	int			i;

	/* Sort entries by usage and deallocate USAGE_DEALLOC_PERCENT of them. */

	entries = palloc(hash_get_num_entries(pgss_hash) * sizeof(pgssEntry *));

	i = 0;
	hash_seq_init(&hash_seq, pgss_hash);
	while ((entry = hash_seq_search(&hash_seq)) != NULL)
	{
		entries[i++] = entry;
		entry->counters.usage *= USAGE_DECREASE_FACTOR;
	}

	qsort(entries, i, sizeof(pgssEntry *), entry_cmp);
	nvictims = Max(10, i * USAGE_DEALLOC_PERCENT / 100);
	nvictims = Min(nvictims, i);

	for (i = 0; i < nvictims; i++)
	{
		hash_search(pgss_hash, &entries[i]->key, HASH_REMOVE, NULL);
	}

	pfree(entries);
}

/*
 * Release all entries.
 */
static void
entry_reset(void)
{
	HASH_SEQ_STATUS hash_seq;
	pgssEntry  *entry;

	LWLockAcquire(pgss->lock, LW_EXCLUSIVE);

	hash_seq_init(&hash_seq, pgss_hash);
	while ((entry = hash_seq_search(&hash_seq)) != NULL)
	{
		hash_search(pgss_hash, &entry->key, HASH_REMOVE, NULL);
	}

	LWLockRelease(pgss->lock);
}