Disable cache by default (#1257)

Disabling cache by default leads to better performance when compression
and encryption is disabled.

Result of read bench tool (data is not encrypted or compressed)
1. With 1 GB cache:
       	Average read speed              : 41.3833 MB/s
      	Total bytes read in 1 minute    : 2.4 GB

2. Without cache:
	Average read speed		: 58.86 MB/s
	Total bytes read in 1 minute	: 3.3 GB

This PR also fixes a small issue with the boolean condition in table.Open().

options.go

@@ -131,7 +131,7 @@ func DefaultOptions(path string) Options {
 		KeepL0InMemory:          true,
 		VerifyValueChecksum:     false,
 		Compression:             options.None,
-		MaxCacheSize:            1 << 30, // 1 GB
+		MaxCacheSize:            0,
 		MaxBfCacheSize:          0,
 		LoadBloomsOnOpen:        true,
 		// The following benchmarks were done on a 4 KB block size (default block size). The
@@ -547,8 +547,13 @@ func (opt Options) WithChecksumVerificationMode(cvMode options.ChecksumVerificat
 // WithMaxCacheSize returns a new Options value with MaxCacheSize set to the given value.
 //
 // This value specifies how much data cache should hold in memory. A small size of cache means lower
-// memory consumption and lookups/iterations would take longer. Setting size to zero disables the
+// memory consumption and lookups/iterations would take longer.
+// It is recommended to use a cache if you're using compression or encryption.
+// If compression and encryption both are disabled, adding a cache will lead to
+// unnecessary overhead which will affect the read performance. Setting size to zero disables the
 // cache altogether.
+//
+// Default value of MaxCacheSize is zero.
 func (opt Options) WithMaxCacheSize(size int64) Options {
 	opt.MaxCacheSize = size
 	return opt
@@ -615,7 +620,7 @@ func (opt Options) WithMaxBfCacheSize(size int64) Options {
 	return opt
 }
 
-// WithLoadsBloomOnOpen returns a new Options value with LoadBloomsOnOpen set to the given value.
+// WithLoadBloomsOnOpen returns a new Options value with LoadBloomsOnOpen set to the given value.
 //
 // Badger uses bloom filters to speed up key lookups. When LoadBloomsOnOpen is set
 // to false, all bloom filters will be loaded on DB open. This is supposed to

table/table.go

@@ -380,7 +380,7 @@ func (t *Table) readIndex() error {
 	t.estimatedSize = index.EstimatedSize
 	t.blockIndex = index.Offsets
 
-	if !t.opt.LoadBloomsOnOpen {
+	if t.opt.LoadBloomsOnOpen {
 		t.bf, _ = t.readBloomFilter()
 	}
 
@@ -513,7 +513,7 @@ func (t *Table) DoesNotHave(hash uint64) bool {
 	if t.opt.BfCache == nil {
 		// Load bloomfilter into memory if the cache is absent.
 		if t.bf == nil {
-			y.AssertTrue(t.opt.LoadBloomsOnOpen)
+			y.AssertTrue(!t.opt.LoadBloomsOnOpen)
 			t.bf, _ = t.readBloomFilter()
 		}
 		return !t.bf.Has(hash)